IMO punching with weights might increase handspeed a bit but will not help with power one ounce. Due toi gravity the weight is trying to go down not out hence your muscles are fighting to keep the weight horizontal...this not not increase power. Like someone else said use reistance bands anchored to a point and punch through using your entire body. That shadowboxer bit of kit is useless and doesn't reuire you to move your whole body through the punch.

Best bet for developing power is to use resistance bands anchored to a low point and punch through using your entire body then drill this like fuck into the heavy bag. Your power will grow by a huge amount.

Forget punching with weights, for power development its futile and more that likely will cause you shoulder probs.