Reduced friction/cohesion/duration of impact, which in turn reduces torque.
It's a good point and should be designed into the new helmets to help protect from neck injury IMO.
Note: I would like to see shock absorbtion added to the exterior of the helmet, hard shell in the middle, it might be possible to use round gel balls mounted to spin freely on the exterior and accomplish both applications. By doing so with smaller, freely rotating balls, even the torque created from more of a head on collision would be reduced IMO.