The lower body mechanics for developing rotational energy used by Barry Bonds to hit a baseball 400 feet is almost identical to those mechanics used by John Daly to hit a golfball 1000 feet. It is important to remember that rotation around a stationary axis is generated from the ground up. The rotation of the hips and torso comes mainly from the action of both legs . Rotation of the hips is not generated from the actions of the arms or swinging of the shoulders. In fact, just the opposite is true.
Rotation is the power source of a great hitter.
But, generating power is not worth a dime if the batter's mechanics cannot efficiently transfer the energy into bat speed. Yes, we do hit off of a fairly straight front leg. The batter's lead knee will be well flexed and pointing at the plate as the swing starts. Then, the lead knee and leg rotate and straighten to drive the front hip in an arc around toward the catcher at the same rate the back leg rotates the back hip around toward the pitcher. So yes, at contact, full rotation of the hips will have occurred and thus the front leg will have fully extended .