My understanding is: Early shoulder turn + continuous racquet head acceleration.
Thinking "pulling your hand back early" is a very bad thing, and could ruin your forehand.
It is amazing to see pros still having his/her racquet in both hands when the ball bounces. But since his/her shoulder had already fully turned, the racquet can start accelerating and still hit the ball in time.
Serve & volleying with a Wilson Pro Staff Six.One 95