Power playing occurs in role playing, and it's when you control someone else's character, as well as your own, without the other person's permission
Say that I play Bob, and someone else is Susie.
Bob watched Susie as she started to walk away. he ran up to her, the girl of his dreams, and yelled, "SUSIE, DON"T LEAVE ME!" She turned around, and glared at him, with resentment in her eyes, before walking up to him, slapping him, and then walking away again
As you can see, if this were a post, I'd be typing Bob's actions, as well as Susie's. The fact that I'm typing Susie's actions is power playing