It is mainly important for the defense on the powerplay. When you contact the puck, you should contact the ice one inch behind the puck. It is this impact which causes your shaft to bend, thus giving your shot power. If you are really strong, then you can contact the puck about 3 inches behind it. This will cause the shaft to bend even more, causing an even more powerful shot.
Some people like to slide their bottom hand down a little as they take the slap-shot.
Like in the wrist-shot, the height of your follow through will determine how high the puck goes. Also, like in the wrist-shot, the most important thing with this shot is the weight transfer. This is where you get a lot of the power. Your weight will shift from your back to your front leg. Click here for Tim Falconer's Six Stages of the Slapshot .