"Transfer Your Weight to a Stiff Front Leg"
Subject: Power source of the great hitters.
Linear "weight transfer" to the front leg is helpful in developing inline energy for some athletic movements such as pushing a shot or medicine ball. Neither is intended to have a circular trajectory. Boxers stride and transfer their weight forward as they deliver a blow straight out to an opponent's mid-section. Average hitters also use simular mechanics as they attempt to accelerate the bat head (in an arc) by transferring their weight and pushing straight out with the top hand. But linear mechanics are inherently inefficient in developing angular velocities.
The power source of the great hitters is derived from "rotational forces around a stationary axis." Rotational energy is very efficient in accelerating objects in a circular path. 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. That's right, the power source for the "baseball" swing is basically the same as the power source for the "golf swing." In both swings the energy is developed from "rotation around a stationary axis."
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. Using both legs (pushing in opposite directions) to rotate the hips is what keeps the center of rotation stationary. 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. Of what use would a 1000 horsepower engine be if the transmission slips? --- You can not efficiently transfer rotational energy into angular bat head acceleration with linear mechanics.
NOTE: Yes, we do hit off of a fairly straight front leg. But we should not think of it as "throwing our weight onto (or against) a stiff 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.
--- When the arms reach full extension and the bat is sweeping past the pitcher, the pulling action of the bat's momentum can pull the body forward enough that the weight on the back foot becomes light or may even clear the ground. But this comes from the bat's reaction - not casting weight forward.