There is no part of him that thinks he has anything worthwhile to teach. When Wrench looks at himself, he doesn't see a man with skills or knowledge or the capacity to pass any amount of that onto someone else. The abilities that he has seem mostly the burdensome kind, and he's not as eager to impart them onto someone else as the man who gave them to him was in teaching him how to kill. So, he doesn't consider himself much of an instructor. Doesn't really think that Arthur would be interested in learning from him. But when the man repeats that sign almost like it's a secret delight, there are about a million things Wrench wants to say to him simultaneously.

He can't, of course, but when his mind gets ahead of his hands he grips the other man by both arms, like he's got to impart his giddy energy onto Arthur somehow. Then he takes his hands back and nods both fists into the other man's to show that yes, that's indeed the sign. He'd like to explain the particulars of why; the rather demonstrative and visceral nature of that open circle and what it represents, but he lets it pass for now.

Now that he knows Arthur knows the alphabet, Wrench is happy to use that instead. It feels clearer than writing letter by letter with his index finger, and he hopes it feels that way to the other man too.

Your handwriting too.
He refused to say much. Said he needed your permission.