Powergaming is a term used to describe forcing your RP onto another person. An example of this would go like this:
"/me jumps down from the ledge and shoots all 4 people in the room with perfect aim"
This is powergame because the person wouldn't have allowed anyone to react- because they shot them, and because the actual /me doesn't leave room for interaction. Changing the /me to "/me jumps down from the ledge and attempts to shoot all 4 people in the room" would not be powergaming, because it allows others to respond to the /me, and doesn't indicate the person doing something instantaneously.