>>> My son's college coach wants him to swing down on the ball to create more backspin. He wants a pronounced lead arm extension at contact swinging down on the ball. Is the correct way. He is having a lot of trouble with this. I took him to his old hitting coach over Christmas break and the hitting coach said my son's college coach is crazy. Please help us! <<<
I agree with his old hitting coach. Swinging downward and extending the lead-arm to contact are both detrimental to good hitting. Let us first address the role of the lead-arm in a high level swing.
In order to attain maximum acceleration of the bat approaching contact, the bottom-hand must apply a rearward force on the handle as the top-hand applies the forward force. To better illustrate this, I will use the analogy of loosening a nut with a 4-prong tire wrench. -- In order to loosen the nut, you would push up with your right-hand while pulling downward with your left-hand. Pushing up or pulling down with both hands would apply no torque on the nut. The forces must be from opposing directions to torque the nut loose.
Let us use that analogy in the your swing. -- Starting your swing with a bent lead-elbow and then extending your lead-hand forward as you drive forward with your back-hand would apply little torque to accelerate the bat-head to contact. However, if you maintain the bend in your elbow (not extending the arm), the rotation of your lead-shoulder back toward the catcher will provide a rearward pull on the lead-hand you need to apply maximum torque.
I will place below a few video clips that illustrate this better than I can describe it with words. Note that in the "Bonds/Burrell" clips, their lead-arms are being pulled rearward rather than extending forward. So as not to change the subject matter, I will address the "swing down at the ball" issue in a later post.
Rose -- hook in handpath
Rose -- hook handpath w/ BHT
Burrell & Bonds BHT mechanics