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.
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