When a player bends his hockey stick when taking a shot, it essentially turns the hockey stick into a spring storing energy. When the spring is released (when the stick unbends and returns to straight), the energy is released and accelerates the puck effectively increasing the speed and power of the shot
A good example of flex is when taking a slap shot, the technique is to have the blade strike the ice a few inches before the puck to load the spring of the stick prior the stick striking the puck, as the stick moves forward in its swing it hits the puck and snaps forward from the bend into the straight normal position again. This same bending and snapping forward also happens with the wrist shot etc.
You want the stiffest flex stick that you can flex completely to take full advantage of the stick recoil as it snaps the puck forward. If the hockey stick is too stiff then the player won’t be able to bend it and get the extra accelerating effects. If the flex isn’t stiff enough the hockey stick will bend too much and the accuracy of the shot will be lost.
