I think grab breaking is a great idea, it could be a toggleable setting for mod makers that want to create something new and different. It wouldn't mess with existing mods, but for the next generation of toribash mod making and playing, it would be a cool addition. Supported.
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The mechanics would go something like this:
Hands would have an amount of health just like joints do and the grip break threshold would be how much health your hands have. The health would only be active when the hand was in grab mode. The health would regenerate just like joint health does but if your opponent does enough damage at one time the grab would break and that hand would let go. As soon as the hand is set to grab again, it would be set back to full health and able to grab again.
This would add a dynamic aspect to fighting, so that you could break an opponents grab, but you would have to act fast to take advantage of it or they could just as easily re grab and bring their grip back to full health. It would mean that skilled players could fight over grabs, breaking and re grabbing in an attempt to control each other.