Since Chikusa's holding still, and didn't say anything about Ken holding back at all, Ken's going to take that as an open invitation to start fucking his mouth in earnest, rocking his hips while keeping that hold on the back of Chikusa's neck. (He actually is paying attention to Chikusa while he does this; he doesn't want to have to explain to Mukuro-sama that he suffocated Kakipi with his cock. But he's assuming that since Chikusa suggested this idea that he's able to handle it, unless something goes drastically wrong, so he's not particularly worried.) And he groans again, louder this time.

When Chikusa grabs his jacket, he looks down, the movement of his hips slowing as he takes in Chikusa's expression. And then...he grins, and goes right back to what he was doing. With no noticeable change to the sounds he's making.

So, the question is - did Ken not understand, or is he being a sly dog and pretending he didn't understand? There's no real way for Chikusa to tell, and the question is whether there's anything more he can do about it.