This is that moment of inspiration where the protagonist realizes the solution to his groups (or mankind's) seemingly hopeless situation. Typical hopeless situations include the superior/invincible enemy force, global disease outbreak, zombie apocalypse, alien invasion, etc
In the final hour, after all conventional ideas proved totally useless, the camera zooms in on the protagonist making his "oh my god. That's it!" expression and explaining the simple solution for defeating the enemy force.
The solution does not take development. It is simple enough and available enough that once the hero realizes it, he can piece together a version of it within a day.
The solution is not the result of research and development. It is just a moment of inspiration, usually resulting from the hero noticing some minor detail or from some supporting character making a casual statement at just the right time.
(EX: Independence Day. David's father warns him that he'll "catch a cold" giving David the idea to infect the alien mothership with a computer virus)
Common characteristics of this trope:
1. The will/won't work statement. Involves either a skeptical sidekick questioning "are you sure this is going to work" or (more often) the expert/scientist working the problem who didn't catch this stating "My God he's right! In theory it SHOULD/COULD work."
2. If he's wrong he dies. The hero volunteers to test it himself. If the theory is wrong, certain death will ensue. Examples in include the experimental vaccine he says "test it on me" or the experimental weapon he carries into the climax battle against the aliens/zombies/enemy himself.
3. The all points bulletin. Now that the simple, easy to use solution to world destruction is tested, someone in charge heard commanding subordinates to spread the word to all the other teams/countries. This signifies the point we "turn the tide" in the battle for survival.
EX: Independence Day, General Gray, "Get on the wire to every squadron around the world. Tell them how to bring those SO Bs