I put some duct tape over the slave's mouth to make sure he can't breathe through his mouth - and then start to smother him by pressing my feet in socks firmly on his nose! He's really well trained and doesn't try to move - at least not until he's getting really afraid. The longer I keep my socks pressed on his nose, the more you can see his struggle in the shaking of his body. And even in the breaks I grant him he doesn't get enough oxygen through only his nose to fully recover. I love to look into his eyes - seeing his desperation, his fear but also his commitment to please me! Later I make it even harder for him by sitting on his stomach while smothering him - let's see how far I can push him!
