I'm still interested if the mythical "real" facesitting knockout exists. The last time I was properly facesat, the lady decided to play with her iPhone timer and my "record" for holding my breath was about 2m30s. That's very unusual for me, because usually I'm struggling after about a minute, but sometimes if the atmos is right in the session, I get this kind of zen relaxedness and can hold my breath for ages. It was the same when sessioning with Robin of Les Femmes Fatales - she had this way where I felt so relaxed that panic you get when you need air (*) never materialised.
To cut a long story short, I think virtually all facesit knockouts in videos are with similar guys who can hold their breath a long long time and "play dead".
(*) = BTW the "panic" you get when holding your breath is not due to a lack of oxygen in your system, rather a buildup of CO2 in your lungs which is actually the trigger molecule for taking breaths. So I guess feeling relaxed = lower metabolic rate = slower release of Co2 into the lungs.