>>> Hi Jack, My questions is in regards to follow thru, my daughter has good rotaional skills however she exaggerates her elbow up in her lead arm after contact. We practiced with the bag as you recommended but was never clear what to do with hands and arms after the point of contact. Should they be fully extended after contact? <<<
The "follow through" phase of the swing occurs after contact and the ball has left the bat. We must keep in mind that during the bat/ball collision, the bat is only in contact with the ball for 1/2000 sec and the bat only moves forward about 3/4 inch before the ball is gone. Once the ball is gone, continuing to expend energy is just wasted energy that should have been more efficiently used earlier in the swing.
The average hitter's swing mechanics accelerate the knob but leaves the bat-head trailing behind the hands too far into the swing. The bat-head does not reach maximum angular velocity until it has swung well past the optimum contact area. This is why the batter has little power to the opposite field and most of the best hit balls are usually pulled foul. In an attempt to attain maximum velocity, these hitters are still driving on the handle well after contact in the follow through phase.
Great hitters have swing mechanics that generates acceleration right from initiation and attains maximum velocity in the optimum contact zone. The way great hitters depleted their rotational energy by contact is similar to the way a lumberjack sinks his axe into a tree. The lumberjack depletes his energy accelerating the axe. It is the momentum of the axe that sinks the blade into the tree -- not the driving of the arms. The lumberjack's arms and hands are almost motionless at contact. -- If the tree were not there to stop the axe, its momentum would pull his arms to extension during the follow through.
Note: The same is true with a batter working with the "heavy bag." The batter should have also expended all energy and be motionless at contact. The bat/ball collision only depletes a portion of the bat's momentum. The balance of the bat momentum powers the follow through. -- Below is a video clip showing the contact position of WCWS hitters and post discussing the correct use of the "heavy bag."
WCWS - Contact Position
Confusion/heavy bag drill