Flinches and freezing are, in fact, pretty legible to Keltham as the initial real reaction followed by her brain undergoing an internal collision between everything it's trying to automatically do and deliberately not do.
There are ways to try to work past this, if you happen to already be trained in all of them; you can imagine careful things that might be said in words, to unravel this. Carissa being who she is, their relationship being what it is, it seems that he is to try to brute-force this and cheat, for it is easier to brute-force pain than pleasure.
It is easier to do all this and push past his own internal screaming if he can tell himself that he is hurting Carissa for a reason, and reasons like that are hard to come by if you are not able to just make them up.
Keltham takes off his shirt.
"Carissa, I need, for the sake of my own sexuality, to have some idea of what this is actually doing. Part of me thinks it doesn't do anything. I'm going to apply one droplet of #3 to my own nipple, with the neutralizer ready to hand. I am checking that you think this is safe for me, and allowing you a temporary exemption until the next time I hurt you, to argue that you should not be in your chains while I do this."