This is called the run-in, and can be done by both heels and faces alike — but there's nothing like the crowd going wild when a face runs in to save an overmatched friend.
Many tag team matches contain a variant of the Big Damn Heroes called the Ricky Morton, where in one member of a tag team is beaten down, and after a period, tags in his fresh partner to the cheers of the crowd. The person giving the tag is the Ricky Morton, the one receiving the tag is going to get a Big Damn Heroes moment.
Subverted at WCW Fall Brawl 1996, in which Team WCW was to fight the NWO in a WarGames match. Sting was originally supposed to be part of Team WCW, but then the nWo announced Sting had joined them (while running around with a fake Sting), and would be part of their team instead. Team WCW believed that Sting had actually betrayed them, and went in a man down, and thus were getting their clock cleaned by the nWo. But who should arrive as WCW's fourth man but the real Sting! Sting beat up the entire nWo team all by himself — and then walked out of the match, angry because the WCW guys (including his own best friend Lex Luger) distrusted him enough to believe he'd actually join the nWo. WCW lost the match when Luger submitted to the fake Sting's Scorpion Deathlock.
At ROH Death Before Dishnor IV, Team ROH (Samoa Joe/Ace Steel/BJ Whitmer/Adam Pearce/Bryan Danielson) was looking at a surefire loss to Team CZW (Chris Hero/Claudio Castagnoli/Necro Butcher/Nate Webb/Wrestling/Eddie Kingston) in Cage of Death after Bryan Danielson clipped Samoa Joe's knee, taking both out of the match, leaving a TWO man advantage for Combat Zone Wrestling. Then, in comes Homicide as the unofficial sixth ROH member. Still down a man, Homicide pinned Nate Webb to win Cage of Death for ROH.
During the showdown between John Cena and Ryback, The Shield arrives to beat the crap out of Ryback as usual. But Cena (whom Ryback had accused of not saving him from them) comes back with a steel chair and drives them away with it. Thus, he proved him wrong by coming to his aid that night.
When he's a face, and the newest monster heel is in the ring destroying members of the roster, The Undertaker tends to perform this.
Kurrgan actually had this as his gimmick when he started his career as the Acadian Giant in the Atlantic Grand Prix Wrestling promotion in the Maritimes in the late 1980s, which makes sense, given that he's 7 feet tall and 350 lbs.
On the September 9 (taped August 29), 1998 episode of ECW Hardcore TV, Tommy Dreamer was in a gauntlet match against Lance Wright'snote Former host of the "Hype Central" segments who had become part of the pro-WWF side in 1997-1998 before that feud faded out stable of "Dastardly" Danny Doring, Jack Victory, "The Canadian Tiger" Mike Lozansky and the Equalizernote Not the same guy who had been Kevin Sullivan's dyslexic"brother" Dave Sullivan in WCW., with the intention of getting revenge for New Jack, who Victory had put out of action with a ripped groin. Dreamer was fighting against all four guys as best he could until it became obvious that he was outnumbered. With Dreamer down, Doring got on the mic and talked about how he's the best looking guy in the locker room, the most hardcore and the best dancer, and decided to prove it by dancing, with his theme, Soul Coughing's "Super Bon Bon" playing. Then "Let Me Clear My Throat" by DJ Kool hit, and Super Nova ran out with a STOP sign followed by The Blue Meanie with a garbage can full of weapons. Joey Styles called them, "Blue Jack and Novastafa!" Meanie and Nova emptied out the weapons, which included a tennis racket, a chair, a Captain America shield and more. Dreamer DDT'd Victory on the shield, and found among the weapons a bowling ball bag. He placed the shield on Victory's groin and dropped the bowling ball bag there for the pin. After the match, Meanie and Nova dragged Wright into the ring and set him up in the corner with his legs spread. Dreamer then debuted the bowling ball to the groin spot that Al Snow would later use in WWE, with Joey saying, "Steerike!" Dreamer's music, Alice in Chains' "Man in the Box" as he, Meanie and Nova did New Jack's "X" sign, before "Let Me Clear My Throat" came back on and Dreamer, after a brief moment of reluctance, danced in the ring alongside Meanie and Nova.
During the main event of September 16, 2013, Daniel Bryan was going to win against Roman Reigns of the Shield...until Randy Orton interferes and viciously attacks the former. It looked like Orton was going to do the same thing he had done to The Miz earlier that night (trying to break his neck)...but just then, the faces of the WWE rush out and aid Bryan in fighting off Orton and the Shield. They give Bryan the honors of finally finishing off his foe and hoist him onto their shoulders, celebrating the victory.
The Shield themselves had done this on April 7, 2014, saving Daniel Bryan from getting screwed out of his championship again by Triple H.