There's almost an art to it: the angle of his hand, the speed of the strike, the amount of force. Picking just where to land it. He can create and even shade over Clark's skin, or raise patterns. He can - and does - nudge Clark's knees wider apart, help him shift his hips up higher, switch hands to continue to hold him at the opposite hip. Bruce slaps him hard, right against the cleft of his ass held open just so, knowing it'll collide with the sense-memory of his mouth, and shoot straight up his spine. He grips hard on an area he's worked the most, forcing Clark to feel the continued burn of it, like a long, continuous moment of impact.

This position is somewhat odd for it all. But it avoids any awkward mimicry of role playing punishment; Clark isn't over his knees. Bruce isn't standing up behind him with a whip. It's an introductory that's as organic and almost-accidental as everything else they've stumbled into, and it's shockingly beautiful. Clark and his back exposed and flexing, flinching, his bound arms cross-crossed in black, his ass tipped up into Bruce's hands.

The physical exertion of hitting him has made his erection flag, blood flow directed elsewhere, but arousal is still practically choking him. It's held there in an iron grip, unable to go higher or seek fulfilment. After Clark pushes back, cries out in a particular way, Bruce leans over him to press a kiss against the center of his spine, both hands firmly on him. Approval, not apology.