The way Ianto looks when he twists back to look at him takes his breath away for a moment, makes him want to throw down the strap and just fuck him, take his breath away too. He still wants to take this further, though, so he gives Ianto a grin and keeps right on hitting him, varying his angle and placement to cover everything from Ianto's upper thighs to just below his waist, so that the pain won't get too intense in any one area but will just grow steadily, evenly covering his skin.
Another pause, then, and he lays the strap down on the bed and leans down beside him, pulling him in for a hot, firm kiss. "How're you doing?" he asks against Ianto's lips, giving him plenty of other stimuli to counter the throbbing in his ass. "Want some more?"