Katrin steps over the slave, shakes her ass and ask him "do you like, wht you see?" "mmh year!!"
She teases him more, comes down with her ass until it touches nearly his nose, only for getting up again.
After some of these teasers she plobs down and sits firm on his face and gets heavy and heavier and there is no air.... does he still like it?
He struggles, but under the strong Katrin he has no chance, also because she chains his hands at the bench.
Now she has him helpless and can enjoy the sitting.