Re: BHT vs. THT
In a high level swing, torque at the handle (push/pull of the forearms) is being applied from initiation to contact. This means the hands are applying force to the handle from opposing direction throughout the swing. I refer to torque applied at initiation as THT (Top-Hand-Torque) because the top-hand is applying the 'rearward' force while the bottom-hand is being pulled forward from body rotation. This causes the bat-head to accelerate rearward toward the catcher.
As the bat-head accelerates around toward contact, it is the pulling back of the lead-forearm (and hand) that applies the rearward force. So, THT & BHT just indicates which hand is applying the rearward force. -- Average hitters apply little or no rearward force from initiation to contact. They mainly take both hands forward during the swing. Their handle torque results from driving the top-hand past a slower moving bottom-hand. This produces much less torque than a 'push/pull' action of the forearms.
In regards to your second question, the lead-arm serves as a connection from the rotating force of the lead-shoulder to the knob end of the bat. Therefore, the elbow should remain at a constant angle (like a bent steel bar). When the lead-shoulder rotates back toward the catcher, it will pull the lead-forearm 'rearward'. When you allow a boxed lead-elbow to extend forward approaching contact, the forearm (and hand) will drift forward producing little or no 'rearward' pull.
Yohan, approaching contact, both forearms should be powered by shoulder rotation rather than the extension of the arms. Video analysis of the top hitters shows the back-arm still in an "L" position at contact. The arm does not extend out until the ball has left the bat.
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