The "easy" power in your forehands should come form a rapid rotation of your shoulders/core.
Set up for the ball like Novak in the photos above.
In photo one his feet are at 45 degrees to the baseline, but he has rotated (coiled) his shoulders/core back so that his left side is facing the incoming ball.
In photo 2 he has started to rotate/uncoil with the racquet still pretty far back even though the ball is pretty close already.
In photo 3 his shoulders/core have already fully uncoiled/rotated, with the racquet lagging his rapid shoulder uncoil. The butt of the racquet is still pointed at the ball, but the rapid shoulder uncoiling means that the hand/racquet will continue forward, almost like snapping a whip. There will be some muscular contribution from the arm, especially internal rotation at the shoulder. But the majority of the power will already have come from the rapid shoulder/core uncoiling (rotation) that has already occurred up to this point.
A video of Novak emphasizes this quick uncoiling motion as his source of power:
http://www.youtube.com/watch?v=y8AJYfkJ4hc&NR=1
Coaches Mauro and Kyril emphasize this point, the importance of a contact point well out in front, and some other aspects in these videos:
How To Hit An Easy, Modern Forehand: Pt. 4
http://www.youtube.com/watch?v=-FfIvTg_xoI&feature=related
How To Hit An Easy, Modern Forehand: Pt. 5
http://www.youtube.com/watch?v=2UZ84ueXD5A&feature=related
How To Hit An Easy, Modern Forehand: Pt. 6
http://www.youtube.com/watch?v=1z3bEtDxugw&feature=related
How To Hit An Easy, Modern Forehand: Pt. 7
http://www.youtube.com/watch?v=fP4Omzjg_5Y&feature=related
