the overhand grip still very strongly affects the biceps (anyone with an understanding of kinesiology would know that). the problem was that the underhand grip + rom + plane of motion + intensity technique ie attempting to hold and slow movement excessively, caused his biceps excessive and unnatural strain.
if it was only the combination of underhand grip + great tension (weight), then powerlifters (who use one underhand hook) would be tearing biceps all the time. as i implied, bolton would have tore his biceps years ago, and don't give me the pre-contest nutrient debit bullshit again because ronnie was using an underhook pre-contest pulling 800lbs (twice as much weight as yates) and when you combine the expolsive and ballistic power that ronnie ripped it off the ground (as opposed to dorian's slower, more controlled movement) the bicep on the underhand grip, going on the theories expounded here, should have popped like a balloon at a 6 year olds birthday party.
again, it wasn't the total tension (alone) on the underhand grip that caused the tear, it was dorian's intensity techniques and form + the other factors that tore it.
when he switched to an overhand grip, his rom was actually lower if you compare clips with similar weight being used (there is no point at all comparing clips of him demonstrating without the tension factor and not using weights that challenge him).
note: i'm not questioning dorian's credentials, knowledge or success. i would be a fool to do so, the guy was mr o. i'm strictly and only commenting on what imo caused the man's bicep tear and questioning whether it was necessary to perform the cauing protocol to develop his back (seeing as it was his bicep that ruptured)