Essentially, theres a sort of extrahealth variable. This variable is set to -1.

If extrahealth is equal to -1, then when you take damage past 1 hp, your extrahealth activates, and is set to your musclepower/3. This acts sorta as your health, and will only decrease if you're taking damage past 1 hp. Since extrahealth needs to be -1 to activate, this can only occur once per round per fighter. Additionally, muscle power will only set your extrahealth once, so if your muscle power changes after your extrahealth is set, your extrahealth wont change with it.

For example, if someone is at 48 (out of 2400) hp, have 600 (out of 2400) muscle power, and take 48 damage. Their extrahealth will be set to 200. And will decrease only when you take damage past 1 hp.

Extrahealth also has no effect on pinning whatsoever