Here is a suggested Metarule for any D/s relationship —

Stated by the Dom to the sub: If I ever appear to you to be out of control walk away, do not submit, no matter what I say or do.

It's like relieving the Captain of his duty. You're doing one of two things. You're either committing mutiny or you're doing what must be done. Only your own moral integrity and sense of what constitutes right and true duty can guide you. There is insubordination and there is clear judgment that the one in command has lost touch with his own clear judgment.

Making this Metarule explicit within the relationship cannot guarantee it will never be needed. That is why it is there. When bound beneath the rage of an emergent psychopath it will save no one. No law ever stopped anyone who was intent upon breaking it.