Well bama first sorry to hear about your back trouble. Let off is simply the amount of weight you are holding at full draw. For example if your bow is set at a peak weight of 70 lb., with a 65% let off you would be holding about 24.5 lb, with 80% let off you would be holding about 14 lb. The problem is that your back is still going to have to tolerate the peak weight of 70 lb (or whatever) until your cam(s) roll over to give you your let off.