jimo-
I like to try to keep the weight on the inside of the back foot,although this is not absolutely necessary,it just seems to simplify things and encourage coiling as opposed to swaying.
I also like to use Epstein's torque and numbers drills/descriptions-"wind the rubber band as you stride to balance at toe touch with the weight slightly forward and the front foot open at 45 degrees"
Theoretically,here are some things to test.
Overall the major swing phases(same as throwing up to a point) are:
1-hip cok
2- hand cok
3-coil/rotate into toe touch (wind rubber band,see above)
4-stretch
5-swing
I believe that Bobby Jones description of the weight shift/body coil sequence also applies to golf(Jones describes this as a way to cure the "reverse pivot" which would be similar to teaching a hitter how to "get off the backside")
Jones found that the weight had to be shifting forward BEFORE the hips began to turn open for good coiling/uncoiling/rotation.
Q:When do the hips turn open in hitting ?
A:hips begin to turn open in phase 3 when you start winding the rubber band.
So,if you have shifted weight back,it needs to start forward (center of gravity forward on line toward pitcher) BEFORE the back elbow starts down to the slot which is synchronized with when the lead leg begins to turn open.Another sign of this phase can be seen on video as the back foot reactively turns back while the stride foot is still in the air-see side view of Glauss or Lugo at youthbaseball clips for example,Nomar shows it often too).
Q:how is weight shift used/harnessed?
A:forward weight shift is blocked during the phase4-stretch of the swing,the part Epstein describes nicely as "drop and tilt".Prior to this,the body shifts the weight forward and coils the body and touches the front toe,then at the right time,the front heel is dropped and the weight shift is blocked with the front foot.This blocking then boosts hip turn to its max velocity,transforms linear momentum into rotational momentum and creates the maximum/last quick bit of coiling(xfactor stretch) of the body which will then unload in the swing phase.
At this time,the center of gravity will need to be somewhere between about the midpoint between the feet (base-where the back foot was and where the front foot comes down) and still behind the weight bearing portion of the front foot.Further forward than this and you are stuck out front,unable to get an adequate stretch/coil.
The control of weight shift timing is via the flex of the back leg/how you "sit on the back leg" as described by Epstein.
I am not used to the 40% measurement.How does this fit with the above description in your opinion ?