Power changes can show up across the whole story. Rocky loses the fight in the first movie, but he regains power over his life and himself. In the original Star Wars, the Empire loses power when the Death Star explodes and the Rebellion gains power by creating some opportunities for later success. Over the course of Amadeus, Salieri goes from being a court favorite to a relatively powerless inmate at an asylum. Mozart becomes legendary.
But power shifts continually in a story, too. A typical scene has three to five beats, and these usually can be interpreted as gains and losses in power. How does a character gain more power in a scene?
Physically. A character may injure or kill an enemy (or enemies). He/she may get an advantage (taking a hill in a battle, pulling out a gun). But a hug or a kiss can also diminish a foe.
Psychologically. Threats, terror, distractions, lures, and arguments can give advantage. Tie the hero’s sweetie to the railroad tracks, and you can own him. (Mustache twirling is optional.)
Through alliances. Sometimes, two weak characters can combine their strengths to defeat an adversary. Votes can shift an election. Loyalty and betrayal can support some characters and undermine others.
By a knock off balance. This is one of my favorites. Generally, this is about changing the topic to something more vital to a character than the current point of conflict. It can be deliberate in a scene, but it doesn’t have to be. Sometimes a character makes an innocent comment that overwhelms his/her opponent. This may be something that suggests grave consequences. But it can be a simple, in the case of a person who is conceited, as a compliment.
By controlling resources. This can be wealth, of course. It can also be the last sandwich on a lifeboat. In a different way, a bribe or the offer of a reward can give one character control over another.
Through information. Think of how secrets revealed, discoveries made, and puzzles coming together can change fortunes. (I love, for instance, how the team on Earth solves Apollo 13’s problem with carbon dioxide. It a triumph of creativity, logic, and understanding.)
Note that, for some of these, the advantage may be apparent, not real. A lie or a mistake or a misunderstanding can swing the odds toward a character and make an opponent vulnerable subjectively, but that can still have real consequences. It can force errors.
And beats can be ironic. If a reader sees a character walk into a trap, the character may feel powerful even as he/she is doomed. Also, each of these dimension of power can be flipped, making a character less powerful, not more. A hero may knock the villain to his/her knees or be knocked to his/her knees by the villain.
One more point to consider is timing. One of the great payoffs in a good story is when the story shifts based on when things happen. In Singin’ in the Rain, the characters raise the curtain on Lina Lamont at exactly the right moment. Both heroes and writers can benefit when they bide their time.