thanks for the replies guys
I still find it hard to understand how Gonzalez can produce so much power with the bent forehand. Is it because he is physically strong?
Most of the power comes from the trunk rotation and not the arm.
both strokes have different advantages:
straight:
-longer lever
-often more stretch in the wrist
bent:
-you can rotate quicker, since the rotational innertia is smaller (like a figure skater brings his arms in when he pirouettes fast)
-you can use internal rotation of the upper arm like a baseball pitcher. first the elbow leads and the forearm lays back(upper arm externally rotates), then the upper arm internally rotates and the forearm moves rotationally around the elbow. this is an extra whip.
explained here:
http://www.revolutionarytennis.com/step8.html
both forehands are good for different types of players. straight arm players need a strong arm and flexible wrist. if your strengths are that go for straight arm.
bent arm needs a very strong core that rotates fast and a flexible shoulder. to get real power with the double bent you need that external/internal rotation move of the upper arm(which is also the driving force in throwing and serving-but with those strokes there is also extension added of course). just going to swing the arm bent won't cut it, the humerus rotation is the turbo boost you need.
players who get that can hit really hard, if not better hit straight arm. the double bent is a very rotational move-in the hips and in the the arm.