Arria,

I train my sub/slaves in this way. "When you offer me your mouth you offer me your power, when you offer me your throat you offer me your life".

Simple put it is a very animal response, when animals submit to the stronger animal they go on their back and allow the stronger to know it could kill them. Same is true here when your Dom has your throat in his hands you know that death could happen and that is where the thrill comes from. I could grab morgan throat in the middle of a fight and she would melt.