[dlam]

I'm sure someone with more anatomy knowledge can give more detail, but when I interlock the left forearm rotation is checked, it just doesn't want to roll.

I'm guessing it is in part due to activation of the muscles supporting the thumb and forefinger of the left hand (separate your index/middle finger in a 'V' as wide as you can to feel what I'm talking about.

Sorry I can't give you more, but at least for me the difference is clear.

I see what you mean. My left FOREARM doesn't want to roll independently as easily with interlocking as opposed to the varnum grip.
This brings up the discussion of how one wants to "turn the club over" . I agree that interlocking grip seems to promote angular hinging motion.