Very sneaky.

On topic: While turning on the speedometer and not pressing "c" help, you also need to think about where you hit.
If you hit in the center of their chest, you're much more likely to break off a pec or lumbar as well, whereas if you hit the forearm/wrist, that's pretty much all that gets dm'ed.

Also think about the state of the joint you're hitting. If you hit a relaxed joint, or a joint moving away from you, there's much less chance of it breaking off than a stiff joint, or one moving towards you.
This is because a relaxed/moving away joint will increase the time of impact, so momentum will be transferred more in time and less in force.
A stiff/moving towards you joint decreases the period of impact, so the force is increased.

This can all be represented by the equation momentum = force x time. And momentum must always remain conserved.

To put this whole thing very simply, more time (longer hit) means less force at once. Less time (quicker hit) means more force at once.

Yeah. I know physics.