He holds his breath as Hijikata moves from his collarbone to his chest because the scar there is the only one with real emotions attached to it--in part because it's the only one he doesn't clearly remember getting. All he knows is that it wasn't good--nothing in that place was good--and trying to remember it leaves him with a bad feeling. But he doesn't want Hijikata to stop, to think he has to, so he brings a hand up and lets it trail along the older man's shoulder in a way he hopes offers some reassurance.