Kitty wants, that her slave comes while he must suffer under her full weight. So she sits reverse on his face and massages his cock with her feet for milking him.
But no way, he cannot come, if he cannot breath and gets crushed under her.
So she moves a little foreward on his throat and continues with her foot-job.
It's also not easy to come with 130 lbs crushing his windpipe.... but with the help of her hands?
He must come, or she would stay sitting on his throat for ever, what martyrium? ;-)