Involuntary contraction is a phenomena often associated with a negligent discharge of a firearm.
Let's look at how this can happen. This is all part of the self- preservation instinct, which we all possess. Anytime we fall, instinct tells us to protect the head. Therefore, we involuntarily place our hand in front of us, GRAB anything close by, or clutch whatever is in our hand. The same thing happens when we are startled.