When I took some Brazilian jiu jitsu classes a few years ago, the instructor showed us the rear naked choke. I think he actually called it hidaka jime. Anyway, he used me as a "victim" for the demonstration. He got his arm around my neck from behind, but I could still breathe. I felt him tighten up his bicep on one side, and I started to panic a little. I got very dizzy, but I felt like I could still step out of it if I kicked him in the right place.
Then he pressed his fist into the other side of my neck and it was like someone turned out the lights. I was conscious one second and gone the next. I don't know how long I was actually out, but the next thing I remember was my instructor slapping the side of my face and pulling me up to my feet. I was still woozy, but not out of commission.