Posted by: tom.guerry (tom.guerry@kp.org) on Mon May 4 15:00:11 2009

That is a good question CFS.

To understand the forces at work, here is some necessary background.

Jack's model is based on how forces are applied to the bat handle, and,
1- you need to distinguish between what Jack calls PreLaunch Torque (PLT), and
2- Top Hand Torque (THT) at "launch" or "initiation".

Initiation/launch in Jack's terminology is when the hand path starts forward motion via it's connection through the shoulders to a turning torso which is powered by turning hips.

Here is an important "passive" model of what happens given the right sort of joint action/muscle tension when a turning torso is connected to the bat handle by a front arm/bottom hand and back arm top hand:

http://www.setpro.com/stuff/rotational_simulation5.wmv

This "passive" model shows that if you increase the force that is applied to the handle via the back arm/top hand (in the model this is done by varying the mass of the back arm), the swing (bathead path) is GREATLY shortened and quickened, which is how you get what Jack calls "early batspeed".

This model is showing the equivalent of "THT at Launch". Joint wise in the "real" (human body/MLB swing pattern)at "GO" this involves tilting of the shoulders as they connect to the turning torso AND forearm rotation, and reciprocal action of the elbows - front elbow up and back elbow down.

All of these actions are turning the bat on a steeper plane than the hips are turning the torso which makes the hands "stay back"/resist turning which controls exactly how/when/where the torso coils and uncoils to drive the bathead quickly to a well matched/spatially early conact point.

Prior to this there is often "PLT" which turns the bat BEFORE the torso turns the handpath forward. The more vertical the bat/steeper the plane, the greater the range of motion for this "resisting" action prior to establishing and swinging in the "matching swing plane".

In this case/earlier phase of the swing, the handle forces do not significantly involve the shoulder action, but just arm/forearm action. This can be thought of as uncocking the bat to give it a running start before "launch" or "GO".

Prior to this phase, you have to set the joints to uncock and GO by cocking tha hands/bat (Jack says "hiding the hands" on pitcher view).

Top hand/back arm is essential in controlling preparation for GO during cocking and uncocking of the bat, and in creating THT at launch.