If you go sliding i don't think the backpack is going to stay in place.
My guess is that this may help with the initial
impact of rider-falling-off-bike. A fall from a relatively short distance (i.e from motorcycle seat to asphalt) can crack a vertebrae or two resulting in paralysis, no? Having said that, I don't understand how this backpack would be a better idea than regular back armor - either inside a jacket or the under-jacket armour type.