She doesn't even really register how powerfully she's gripping him (she'd likely apologize if she did) but what it amounts to is she's channeling all of her passion into just that one little action, so of course it's going to be uncommon strong. She's too worked up to do anything but.

When she notices where he's moving her hand, though, Daenerys smiles slow, playful as she can in the aftermath. Here, the hold she has on him is nothing but careful, tender even, and it's almost testing the waters as she runs her hand up his length then back down it. "Let me," she whispers. It's the least she can do after he managed to bring her to that