The slapshot is the hardest shot in Hockey! Getting up to speeds of 120mph only by the flexing the stick and transfering your body weight throught the shot, therefore youre building up a lot of potential energy right before contact with the puck. It seems that when you see a pro take a slap shot it is all in one swing, with the arms doing all the work and the blade of the stick only hitting the puck. Well, our eyes are decieving because that is far from what actually happens in a true hockey slapshot.
Turns out that as the stick hits the ice first it bends back a little, storing potential energy. Then, as it is lifted, it springs forward off the ice, giving the puck a powerful flick that's actually faster than the hockey player's follow-through would otherwise be. Most of the power in shots for hockey depends on the amount of flex you can put on your stick so when you follow throught the stick will have more flick to it causing it to rise up to speeds of 120 mph. Fun Fact, a stick has to have a force of up to a ton to break!!