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When you are skating you automatically have this speed added to your stick speed as compared to standing still.
The speed of the stick has a lot to do with how strong your forearms and wrists are. You must get the shaft to bend so it whips to project the puck.
As a defenseman you need to get into the habit of dragging the puck quickly inside the dot before shooting (if on the forehand) and skating it there if you are playing the off side point. All of your shooting practice from the point should include getting the puck inside the dot and letting it go.
You also need to use your whole body and not just the arms if you want a hard shot.
So I would say you must combine strengthening your arms and wrist with moving quickly into shooting position. It is hard to say exactly where the weakness is without actually seeing you shoot.