This post discusses the swing mechanics that apply forces at the handle to generate bat speed. Once we understand the forces acting on the bat that induces bat speed, we can better understand the swing mechanics that most efficiently supply those forces.
>> The bottom hand torque is when a hitter pushes his top hand toward the ball and his bottom hand back to the catcher during the swing. But does the hitter literaly applies force with his bottom hand or just keeps his bottom hand still and attacks the ball with his top hand?
Also, what is the exactly is the top hand torque? <<<
There are two forces a batter's mechanics applies to the bat that generates bat speed. (1) An angular displacement rate of the bat-head (bat speed) is induced with an angular displacement rate (circular-hand-path – CHP) of the hand-path. We refer to this as the “Pendulum Effect. -- (2) The angular acceleration of the bat-head from "torque." Torque is applied at the handle when the hands apply force from opposing directions. --- The amount of bat speed generated depends directly on the efficiency of the batter's mechanics to supply these forces.
As stated above, correctly applying torque at the handle is an essential factor in generating maximum bat speed. However, there are two important points to keep in mind to efficiently apply torque. (1) The application of torque 'must not' alter (or straighten) the CHP. (2) Most of the force supplied through to hands to the handle 'must' come from shoulder rotation - not the arms. In a high level swing, the arms serve mainly as linkage from the rotating shoulders to the bat.
To illustrate this, let us use the marked-up clip below of Ken Griffey's mechanics. First, study his lead-arm. Note that that the elbow maintains a fairly fixed angle from initiation to contact. Therefore, it is not the flexing or un-flexing of the lead-arm that is supplying a pulling force on the lead-hand. It is the rearward rotation of the lead-shoulder that is supplying the power. -- The arm is serving mainly as linkage from the rotating shoulder to the hand (and knob-end of the bat).
Shawn states bat rotates same as shoulders -- Brian's video reply
Rose hip/shoulder comparison
Note: See all of Batspeed.com Youtube Videos at - MrBatspeed Page
Now, play it again and study the back-arm. Note that once the back-elbow lowers to his side, it remains back at his side (in the "L" position) during rotation to contact. Therefore, it is not the muscles of the back-arm that is mainly driving the top-hand forward. It is the forward rotation of the back-shoulder that is supplying the real power.
Are the muscles of the back-arm exerting force? Yes, but the force is more to maintain the "L" position that to extend the arms. This is similar to the way you would use your arms to push a car. The arms are exerting a strong force to keep the elbows from collapsing under the pressure, but it is not their extension that moves the car. It is the larger muscles of the legs and torso that is supplying the real power - the arms are their "linkage" to the car.
Leonel, you asked, "But does the hitter literaly applies force with his bottom hand or just keeps his bottom hand still and attacks the ball with his top hand?" -- In order to apply maximum torque, the bottom-hand must apply an equal (but opposing) force from initiation to contact. That rearward opposing force is supplied from the rotation of the lead-shoulder. -- I will address your THT question in another post to this thread.