This keeps them from hyperextending. Alignment is everything. If you get your body in the correct position, the force of the punch explodes through you into the heavy bag. Technique is learned through repetition. Your jab will not come to you overnight, just as the hooks and the straight right will need to be perfected.
Repetition paves the way for instinct.
The more you practice being aware of it, the more it will begin to be your natural way of being. But technique is not everything. Your punches have to mean something else besides whether or not they have been done correctly. This is where gut level punching comes in. Gut level punching is a way of getting more content into your punches.