Heroic Second Wind

It's the end of the movie, the climactic battle royale between The Hero and the Big Bad and The Hero is losing. After a long struggle, the Big Bad finally manages to get through the hero's defenses and score what looks like a decisive hit.

The hero crumples, looking to be in a dire strait indeed. Usually at this point the villain takes the opportunity to gloat a bit and humiliate his opponent verbally or physically, believing the hero to be his to dispatch at convenience.

Often, the hero's loved ones will be watching them getting the beat-down in a horrified state, which adds to the despair of the situation (and is a pretty bad hit to the hero's confidence). In more violent examples, the hero will have wounds that really ought to be fatal.

Then, just when everything seems to be lost, something happens. With a sudden all-out effort, the hero rises, often presaged or accompanied, ready to rejoin the battle and despite the apparently crippling injury just sustained, there's no question at all that the hero's going to end up standing over the villain's smoking corpse — or, more generally, triumph in whatever way is appropriate for the genre.

There are hundreds of ways to do this trope, but some of the most popular ones are:
 * Realize what it is that you are really fighting for if you haven't done so yet.
 * Think of your friends.
 * Your belief in yourself is so strong that you refuse to give up.
 * Your cause is so important, you are simply not allowed to lose.
 * Resolve an inner conflict that has been holding you back.
 * Your magical trinket starts glowing and heals your wounds.
 * Just get really pissed off or do the opposite: Calm your mind and focus.
 * Ask the spirit of your ancestors, your deity or just the universe in general to give you more strength.
 * A loved one is in danger!
 * Realize that you are correct and your opponent is not; being the moral one will also make you superior in combat.
 * Use sheer grit and force of will.
 * On the brink of death you uncover a yet-unrevealed power.
 * Did you make a vow to yourself or a promise to someone else (for example, "I will definitely come back!")? You're not going to break your word, are you?
 * Is your belief in yourself or your weapons so strong that nothing can faze it?
 * Fight the way a person like you would fight, rather than copying someone else.
 * Have a friend or a random kid beg you to stand back up. It'd be rude to let them down, right?
 * A ghost, a Spirit Advisor or some other Deus ex Machina does the same.
 * A Superpowered Evil Side that was hidden until this point suddenly surfaces.
 * Remember what the Big Bad has done and all of his atrocities and then use The Power of Hate.
 * Realize that whatever collateral damage that might come around by unleashing all of your power is nothing compared to what will happen if this villain beats you.
 * The Big Bad makes the mistake of reminding you about all the people that you loved that he killed or harmed. (Bonus points if he obviously enjoys those memories.)
 * Realize that the Big Bad can take all of the power you've been holding back. Bonus points if you've been terrified of what you're capable of up to that point.