Moira, in shiny blue spandex, sits on the slave's face.
She steps over him, cuffs his hands and sits down on his chest.
She moves slowly foreward, until she sits on his throat.
This is a good position for taking more of his air supply with her hands over mouth...
Breathing is difficult with 115 lbs on the windpipe and the hands clutting the rest of oxygen.
The she moves on the face... definitively no more air, but more ass ;-)