I will address this one last time. Those who refuse to accept the physics can find another way to resolve their OCD.

Body flinches, jerks, and other movements DO NOT CAUSE BRAIN INJURY.

For your body to use muscles to cause a level of force than could cause an injury, you would be tearing your muscles and joints or you would need to be making extreme movements like head banging where you throw your head back and forth for ongoing periods causing disorientation and dizziness.

A hard brake action in the car can only generate 1.7 Gs. That is much more that a body jerk can generate.

If body jerks bother you as you go to sleep or during sleep, maybe your doctor can try gabapentin. I use it (200 mgs before bed) to help my body not jerk or tremor during sleep. The jerking disrupts my sleep. I am not worried about it causing damage.

If you have a vestibular disorder, these movements can cause your inner ear to sense the movement. That is not a symptom of injury.