During dormancy, the sap has left the canes and the canes are simply empty tubes of cellulose. Pruning too early in the fall (before the sap runs back down into the roots) cuts some of the nutrients out, so you must be sure the plant is dormant before fall (winter) pruning. The only fall pruning I recommend is if the bush is very tall and the wind will whip it around in the winter, then prune just the very top of the bush. Winter dieback generally occurs from the end of the branches. Pruning removes the available length that can die back before reaching the ground. Pruning a semi dormant plant stimulates growth and sap flow in the pruned region.