B - briefly, one of the ideas is to always use progressive load to help muscle size increase (note it's mostly for size NOT strength). Obviously, you can't just increase weight every time you train forever (or we'd all have like 1200 lb benches by now). So the SD is to allow lighter weights to have some effect so you can start out at lower weights again and then keep adding, going slightly higher than the last cycle. Also, I think it's 7-9 days of SD, a little less than 2 weeks.