Let's say you're writing super hero fiction. The hero of your story is struggling against the villain, but suddenly awakens a power that allows him to best the bad guy!
Problem: you're unsure of how you should introduce that sudden power up. You have two options.
Build up to it
The hero finds out about this powerup and you see him actively taking steps to awaken said power. He struggles, but he does take some small steps forward, hinting that he's going to unlock it soon. Then of course, when all hope is lost, he manages to awaken the power just in time.
Introduce it suddenly, explain later
The first thing you see is the hero awakening the power when he fights the villain. You don't see any buildup to it, you don't even know it's possible. It seems like a DEM. However, later, after the hero has beaten the villain, you see how the hero was able to unlock the ability and the steps he had to take to attain the power. It's like the first option, except in reverse. It didn't come out of nowhere, as there were very small hints before the awakening, even if you didn't directly see what the hero did. Him going into a secluded temple? Turns out he learnt the ability there.
What are the pros and cons of either these options? Are there situations where you should use one of them instead of the other?