There has been a lot of talk about Finger power recently, but what part of your index finger should be applying the pressure to generate the power on forehands.
For me, it's the lower part of the middle bone of the index finger. The exact position will alter slightly according to the forehand shot. i.e. underhand(slightly lower) vs overhead(slightly higher)
What my coach to teach me: Place down your racket on the floor, and put that thumb of yours right on the flattest part of the handle facing the ceiling and pick up the racket with that thumb still right there pointint towards the head of your racket. If you want to get finger power out of backhand, use your pinkie instead of your index and rotate the racket around your thumb with that small pinkie of yours.