Mark, sorry it took me so long to answer your post. I will say that Tom already did an excellent job answering your question. He is a lot more thorough and detailed than myself. However in the interest of discussion here is my 2 cents.
I believe the weight on the back foot is primarily governed by the angle of tilt in the axis of rotation. As the lead arm works up or down to adjust to high and low pitches the angle of the shoulders change accordingly. The shoulders are more tilted for lower pitches and more level for higher pitches. I believe that the weight on the back foot is set involuntary by the body to maintain balance as the shoulders tilt.
Epstein briefly mentions the hinging of the back knee on my "Torque Drill" dvd. He mentions it in the context of; "we're not swinging up in the rotational swing, we're just getting our body back to be level to the ball, not level to the ground." He immediately follows those comments with a reference to how rotational hitters don't groove their swings. He's clearly making a comparison to linear hitting and the fact that rotational hitters always hit with a tilted axis. He also begins the comment with "What the player will wind up doing". Which I understand to mean that hinging the back knee is not a teachable movement that a hitter would drill on.
The area where the hinging of the back knee really comes into play is when a hitter gets fooled on an offspeed pitch. I'm actually impressed that Tom knew about this aspect of Mike's teachings. When a hitter is fooled on an offspeed pitch, hinging the back knee buys time and allows the hitter to keep their weight back and maintain balance. This is the one instance where the hitter will have more weight on the back foot than is customary.
I saw a Dominican player do this a couple of nights ago during the World Baseball Classic. It was very impressive and I made sure to play it back several times for my DD.
I view Epstein's material in it's entirety and don't cherry pick certain aspects of what he teaches. So when it comes to the behavior of the back foot I take into consideration all of his teachings including:
1. The heel of the back foot must come off the ground when the hips are initiated as illustrated in the #2 position of the "Numbers Drill".
2. Collasping the backside is really, really bad.
3. The lead arm/elbow works up for low pitches and down for high pitches and shoulder tilt varies accordingly.
4. The swing must be launched from a balanced position.
5. The hitter's axis is tilted at contact and follow-through.
6. The perfect swing is the adjustment the hitter makes to the oncoming pitch.
Epstein's material has traditionally been geared to distinguishing the differences between rotational hitting and linear hitting. In the future it wouldn't surpise me if he makes some adjustments in phraseology to differentiate himself from all the other rotational material flooding the market.
One thing at a time. Tell me your view of this shoulder tilt. How it happens. In what plane and so on.