Perhaps the mech has a mode inaccessible by the pilot until a specific password is entered. Perhaps the inter-dimensional portal that the characters use for rapid transit is actually a doorway to Hell. Perhaps a certain magic ring can do more than make one invisible.
Often the thing in question was being used by the character in a mundane or straightforward way when untapped potential lay hidden within. Typically that potential is revealed in a surprising way, often by a villain but not always.
At the time that the work is taking place the thing is being used with some regularity, it's just that the current owner/holder is unaware of the thing's other, loftier or more powerful abilities. To clarify, the thing in question serves a purpose outside of its "special" abilities or traits. The sword was being swung, the gun was being fired, and the artifact was being worn for a practical/useful purpose prior to The Reveal. The reader/viewer is usually unaware of the "true" purpose of the thing, but the character is ALWAYS unaware.
A Sister Trope to Chekhov's Gun, Excalibur in the Rust (where the thing has been unused prior to The Reveal), Chekhov's Boomerang (where the thing is used infrequently, or once and forgotten about then used again, and there's no Reveal).
Compare Plot Coupon (when obtaining the thing is part of the work's narrative), Cerebus Retcon (which is what the thing becomes retroactively), and Sword of Plot Advancement (where the thing is obtained late in the narrative of the work). Also see Priceless Paperweight.
Plot Coupon That Does Something is similar to this trope, but the focus is different — in that trope, the "plot device" use is known ahead of time, with the secondary use being a bonus; in this one, the significance is discovered later. It's the difference between the Holy Grail having a helpful effect of healing the bearer, versus your handy Magic Healing Cup turning out to be the Holy Grail.
Contrast MacGuffin (which serves no useful purpose).
- In Attack on Titan, the fact that the audience never gets to hear Levi's last name.
- FLCL: Naota and the "beings" that sprout from the portal in his head serve to drive the plot of the series and other narrative roles, but it's only at the end that the true nature of the portal is revealed.
- Fullmetal Alchemist: Alchemy is widely used throughout the series by protagonists and antagonists alike. It is explained to be a form of Earth energy manipulation. It is only later that the characters learn that Alchemy as they know it is actually powered by an enormous sentient Philosopher's Stone, and gain access to "true" Alchemy by finally tapping directly into the Earth's energy, supercharging their abilities.
- Initial D: The engine in Takumi's Sprinter Trueno blows up, so he receives a new one. The new engine revs much higher and produces more power than the old one, but Takumi is initially unaware of this and drives the car as he always has.
- Naruto: Chakra is only ever discussed as an energy used to power techniques, with any other description being largely technical jargon relating to said techniques. Late in the Fourth Shinobi War it's revealed that all chakra was originally stolen from the God-Tree and then shared with humanity by the Sage of Six Paths. The Juubi is attempting to regain its lost power, while Kaguya is trying to claim it all to create an army.
- In Asterix and the Chieftain's Shield, the shield of Vercingetorix sought by the Romans and Gauls throughout the book turns out to be the shield Vitalstatistix is carried on.
- ElfQuest. The main protagonist, Cutter, carries and uses his deceased father's sword, New Moon, continually all through the first book without even a hint that its pommel can be removed and has a key attached and it isn't until shortly before The Palace War in the fourth book that anyone discovers what the key unlocks.
- In The Castle of Llyr, much is made of the "Golden Pelydryn," an artifact which will reveal the spells in a powerful book of magic. It turns out to be the bauble Princess Eilonwy has been using as a glorified lamp since the beginning of the series.
- In Harry Potter and the Deathly Hallows, Harry's invisibility cloak is revealed to be one of the titular Hallows, three extremely powerful magical artifacts which according to legend, originally belonged to Death itself. Additionally, the entire climax of the second book turns out to be quite important, as Riddle's haunted diary was actually one of Voldemort's seven Horcruxes. Furthermore, a fang from the dead basilisk is used to destroy another Horcrux.
- In The Hobbit, main character Bilbo thinks a magic ring he found is a handy invisibility charm but not much else; it wasn't until The Lord of the Rings that the true nature of the One Ring came to be understood.
- The Wheel of Time: The crystal sword Callandor can only be claimed from its resting place by The Chosen One and is a vastly powerful Amplifier Artifact for the One Power besides. Much later, the Chosen One learns from research that it was designed with a second function that's instrumental in the Final Battle: it can also channel the Dark One's power, which is necessary to re-seal the Dark One in his prison.
- The Planescape setting of Dungeons & Dragons has Sigil, an entire city which functions like this. Every opening, even something as simple as a plank lying at an angle against a wall, is just an opening you step through as normal... unless you have the right key. Then it becomes a portal you can step through to potentially anywhere else in the multiverse. And the "key" can be anything from an actual key or device to a certain word, thought, gesture, musical note, etc. Given that portals and keys can change, figuring out what triggers a portal and where it leads is a major business (as much to know to avoid triggering a portal as the opposite. After all, you don't want to accidentally wind up in hell when you were just looking to get a drink at the local tavern, do you?).
- In Bloodborne, the Umbilical Cords may seem like a rare type of Insight obtaining item but really, consuming at least three of the four you can find will unlock the True Final Boss and secret ending.
- Mega Man ZX Advent: Model A starts off as a mysterious, amnesiac, but otherwise normal Biometal which the main characters Grey/Ashe use to transform into a "Mega Man". After a fight with Atlas (Mega Man Model F) and taking her DNA, Grey/Ashe suddenly gets struck with a message in their inner thoughts, which hints towards someone behind the whole mess and his plans. Fighting other Mega Men reveals more of this, with the final one (against Vent/Aile and Model ZX) reveals the Big Bad Albert's plan of merging all of the Model Ws together to form Ouroboros and use it to reset the world.
- And most of all, after said fight, Model A learns his true purpose of itself, but it refuses to tell Grey/Ashe about it (though Aile/Vent then lectured them about how they make their own destiny, so as to ease their feelings). And in The Very Definitely Final Dungeon, Albert tells Grey/Ashe what Model A is holding in: that Grey is Albert's backup body, Ashe is his legitimate descendant (as opposed to other Mega Men's genetically altered lineage), and Model A contains his backup plan and a smaller copy of Albert's power with Model W to make its Biomatch Albert's successor... In other words, "Model Albert".
- Neverwinter Nights: In the Hordes of the Underdark expansion your character possesses the Relic of the Reaper, a mysterious object that is controlled by an entity called The Reaper that effectively saves you from death whenever your HP hits 0 by pulling you into the Reaper's pocket dimension. It is later discovered that the Reaper serves the Big Bad Mephistopheles, the creator of the Relic, and your possession of it binds the two of you. When the Drow that had bound him into her service orders him to kill you, the order to destroy an extension of himself invalidates the binding and frees him to turn on her. With his plan to escape a success, he reverses the bond, trapping you in his realm of the Hells (which the Reaper's dimension is actually an extension of) while he remains free in the mortal plane.
- Persona 3 and 4: When the game begins, Social Links are introduced as a way of strengthening Personas and have other purposes, none of them related to the combat mechanic. It isn't until the True Final Boss that Social Links reveal their true power: they represent the combined strength of all your friendships and relationships acquired throughout the game, and are the catalyst for sealing Nyx and striking the final blow against Izanami, respectively.
- In RefleX, the player's own ship, the Phoenix, seems to be a pretty standard starfighter, save for its Attack Reflector shield. It turns out to be the 13th ZODIAC, Ophiuchus, awakening its true power after being blasted to seemingly critical condition by ZODIAC Virgo; this gives it wings, upgraded firepower, and a shield that can be deployed forever. The pilot only sees and discovers all of this as he dies from his wounds. ZODIAC Ophiuchus then goes into its personal prime directive: destroying all 12 of the other ZODIAC units.
- Secret of Mana: The rusty sword would be an example where The Reveal happens early in the game, but the player goes through at least the first area and fights the first boss before finding out (unless you read the manual or the intro scroll before sitting down to play). And you still don't find out how to power it up until clearing the next area.
- Soulcalibur: Xiangshua's sword, Krita-Yuga, is actually Soul Calibur.
- In The Sword of Hope, Prince Theo is presented with a modest starting blade and sent on a quest to find the titular weapon. He visits each of the three sages in the kingdom, who profess ignorance of its location but kindly power up the interim sword he's using. Eventually, the old man who raised him powers up the sword a fourth time, revealing it was actually the Sword of Hope all along. When Theo was a baby, he and the three sages divided the sword's power among themselves in order to safeguard it from the possessed King Hennessy.
- No, Thank You!!!: In Ryu's route, Haru makes Ryu childish I.O.U. coupons that he can exchange to make Haru do anything Ryu wants him to do. Haru gives them to Ryu whenever he cooks food for him. Ryu accepts them but rarely ever uses them. It turns out that Ryu has been saving up those coupons so he can interrogate Haru about the whereabouts of his sister and make him answer truthfully about it, because he did say that he'd do anything.
- Red vs. Blue: Tucker finds an energy sword that turns out is an ancient relic of an alien race and its intended use is a key to reveal an ancient alien ship. Tucker thinks that it's super-lame that his precious sword is really just a key. The 'key' function comes back with a vengeance when it turns out to be capable of activating the alien technology on Chorus.
Caboose: No, we came home because the alien died, and because the uh, glowing sword turned out to be a, uh glowing key.Church: Yeah, a glowing key that could still STAB people.Caboose: Right.Church: So it is a sword. It just happens to function like a key in very specific situations.Caboose: Or it's a key all the time, and when you stick it in people, it unlocks their death.