Forehand Balance and Swing Exercise

The exercise below, borrowed and slightly modified from Ray Brown, has been helping me make my forehand better. I'm trying to keep my racquet up until my legs begin to extend. I'm also trying to delay the forward swing of the racquet until my hips then shoulders have begun rotating toward the ball.

I'm trying to do this exercise a couple of times per day (backhand, too) for maybe five or ten minutes at a stretch. Then I try to feel this rhythm and timing in my swings when I hit balls. So far I feel like it's helping. At the risk of falling for the post hoc ergo propter hoc fallacy, I had never hit a 91 mph forehand before doing this exercise, but did afterwards.