Basic Trope: A character reveals a hidden level of power late in a climactic battle.
Straight: During her showdown with Emperor Evulz, Alice is getting beat fairly badly. She then reveals that she was only using about 85% of her power, and powers up accordingly, eventually winning the battle.
Exaggerated: Alice was only using 1% of her full power.
Downplayed: Alice was holding back in order to save her strength for the latter part of the battle.
Alice didn't know the scope of Emperor Evulz' power, and wanted to keep some of her power in reserve to make sure that she didn't use it all up before she could finish him.
Alice's true power could cause a great deal of destruction, but Emperor Evulz has pushed her over the Godzilla Threshold so she can't afford to hold back.
Or, Alice's has perfect control over her true powers, but they would almost certainly kill almost any one, and violate her strict "Thou Shalt Not Kill" rule. She only chooses to unleash them if she's completely sure that the villain in question can take the punishment.
Alice is pretending to be a weaker fighter than she really is so that Emperor Evulz will underestimate her and not fight at his full power to avoid wasting energy.
Inverted: Alice discovers a crucial weakness during her battle.
Subverted: Alice powers up to 100%. There's no appreciable difference in how the fight goes.
Double Subverted: Alice says she's powered up to 100%, so Emperor Evulz does the same. While the Emperor fights much better, Alice doesn't get any stronger. In truth, she was just making sure the Emperor was showing his final form before she powers up for real, knowing he is out of tricks.
Alice has a power increase for every roadblock Emperor Evulz throws her way. Emperor Evulz snarkily lampshades this.
Zig Zagged: Alice says she's powered up to 100%, so Emperor Evulz does the same. While the Emperor fights much better, Alice doesn't get any stronger. In truth, she was just making sure the Emperor was showing his final form before she powers up for real, knowing he is out of tricks — but it still isn't enough, and Emperor Evulz wins.
Averted: Alice fights at full power the whole time.
Enforced: "Look, we've got to please both Alice's and the Emperor's fanbases, so let's have him beat her silly until she reveals she's not using her full power!"
Lampshaded: "I'm not done yet! What I've been telling you is 100% power is really more like 85%!"
Invoked: Alice decides early on she should never go into battle 100% so she'll always have a trump card.
Exploited: Emperor Evulz knows that Alice is prideful, and tries to deal her a critical injury in the opening stage of the fight, before she has a chance to power up.
Defied: Emperor Evulz forces Alice to power up fully at the beginning of the battle.
Discussed: "Let me guess...you're holding back some of your power."
Conversed: "She's not using her full power? What a co-pout!"
Deconstructed: Deliberately handicapping herself means that Alice is killed by a lucky blow early in the fight, with no time to 'switch hands'.
Reconstructed: Alice lives... and fights on through her injury, tapping her own untouched reserves of strength to win the day.
Played For Laughs: The whole thing is a Lensman Arms Race in power increases - the fight goes on for hours with each side increasing their power sporadically.