If Gen actually wanted to keep his arm - he could probably overpower Senku. But why would he do that?

He wanted to be tied down, he was discovering. Not just for the Augur's sake. He let Senku pull his hand into position, head hitting the mattress.

"Oh, fine, if it's for the ritual's sake," Gen teased, his smile playful. "I guess I don't have any real say in how it goes when I'm trussed up like this, do I?"