Each time he moved more roughly, she'd utter a low groan and retaliate with another bite - with neither of them willing to concede dominance it was a battle of another sort between them. Although it wouldn't be just his back that would be a virtual storyboard of how insane he got her for him - he was going to have bites all over his body - in fact, if anyone else were to see him they might think that he got into a battle with a wildcat or something! And Tiara was just pushing him further - she already lost all sense of her control, he's not allowed to have any himself!