Situation and confidence for me. Its hard to really objectify shot power. For placement and in situations where Im out of position or simply dont need power then I ease back. When confidence is high, Im warmed up and I have a good 'feel' for the ball then generally I start hitting harder.
My game turns pears when I start trying to hit too hard. So if Im struggling to get it in I ease off a go for placement. Even to the extent of turning into a pusher and forcing my opponent to try and hit harder, bringing out unforced errors.
Also, when I play players that are better than me, or when my opponent is high in confidence and hitting the ball hard, I find I subconciously feed on this and match their game.
Its strange that in tennis my best game comes out when Im playing better opponents. Rising to the occasion I guess. On the other hand, against lesser opponents I tend to play to their level. Frustrating for me.