METHOD 1 – The German medical student technique
This one is so safe, you probably could almost do it alone. There are no restraints involved. The problem is that it involves passing out from a standing position, which means falling down on the floor. This technique was described by German scientists in the 1990s, who used it to study fainting in volunteers, who were all German medical students. The research participants do it to themselves. But it is done on a heavily padded floor, with a helper there to make sure that people fall backwards rather than forwards. Doing it without these safety features will most likely cause injury from falling to the ground.
There is a great video available of the original German experiments, which I previously posted here:
https://fetlife.com/groups/73684/group_posts/13006206
This method is quite interesting scientifically. Although it involves breathing techniques, it is actually more like a blood choke, in that it is based on a lack of blood flow to the brain. The blood still has plenty of oxygen (the normal amount), but you are "tricking" the body into reducing cerebral blood flow to such a low point that there is not enough oxygen being delivered to the brain to stay conscious. The technique combines two different tricks that both reduce cerebral blood flow through different mechanisms.
First, you get in a crouch with your knees bent, and you hyperventilate for about a minute. Hyperventilation does not really change the amount of oxygen in the blood, but it does strongly reduce the amount of CO2. You are essentially blowing out CO2 at a much higher rate than normal. CO2 is used by the body as a signal for dilating and constricting blood vessels. If your CO2 increases, your body thinks that you are not getting enough gas exchange happening in your organs so it dilates the blood vessels for more blood to flow. If CO2 falls, the opposite happens and the blood vessels constrict. So if you hyperventilate for about a minute, you constrict the blood vessels as much as they can go. This is not enough to knock you out normally, but it can get you close to passing out, so close that the second part of the Valsalva maneuver will push you over the edge into unconsciousness.
For part 2, you suddenly stand up, and then perform a Valsalva maneuver. This means that you try to exhale, but you have your throat closed so that no air actually comes out. Having trouble understanding what I mean? Here's a helpful hint – it's exactly what you do when you are on the toilet for a bowel movement, and you've got a very large turd that you're trying to pass but having trouble. So you "bear down" on it. When you do this, you raise the pressure inside your chest quite a lot. Through a complicated sequence of events involving biological sensors that monitor your blood pressure, this causes your heart to relax and pump less blood. Ordinarily this would not be enough of a loss in blood flow to cause unconsciousness, but combined with the hyperventilation and suddenly standing, it's enough of a combination to cause fainting in most people. Congratulations, you have passed out! You will return to normal within 10-30 seconds.
METHOD 2 – Carotid choke
This is the simplest, safest, and most effective way to make a partner pass out. You use constriction to block blood flow in the carotid arteries. The loss of blood pressure to the brain results in insufficient oxygen to brain cells, and unconsciousness quickly ensues. If you don't make it all the way to a pass-out, you will still feel light-headed, have loss of vision ("gray out"), and often hear strange noises - sometimes described as ringing, but I think they sound more like something out of a horror movie. When pressure on the neck is released, consciousness should return in 10-20 seconds. This mechanism is basically the same no matter how the constriction is achieved – manual choking, garotte, ligature, hanging, etc. Probably the safest version is the "rear naked choke" - a judo/jiujitsu move that you can learn to do on youtube. The particulars can vary a little according to the method of constriction. For instance, with hanging, sometimes one side of the brain is more constricted than the other, and it can take a long time to pass out. This can be dangerous - if you hang someone for one minute and they still haven't passed out, it could be that one side of their brain is highly deprived of blood and the other isn't, so they're still awake - you might want to end that hanging even if they're still awake. Have them hold something (like a tennis ball) in both hands, and when they drop one, it's a good time to stop.
These first two methods are both based on loss of blood flow to the brain, and they both work very quickly. The first method uses the body's own regulatory mechanisms against it, and the second is more of a brute force approach. Both of these methods are fairly safe (not entirely without risk obviously), as the total amount of time that the brain is deprived of oxygen is quite short, and the rest of the body (i.e. the heart) is barely deprived at all. There are some theoretical concerns about pressure on the neck causing vagus nerve stimulation which can slow the heart and theoretically stop the heart – most famously raised by Jay Wiseman. That's a hotly debated topic and I'll save it for another note in the future, but simply put I think it's a very very remote risk for ordinary choking play among healthy people. For the rest of this note, I'll talk about ways to pass out by depriving the body's blood supply of oxygen, even though blood flow to the brain is still normal. These methods may be a bit more dangerous, because besides the effects on the brain, prolonged lack of oxygen can interfere with the heart's function, leading to a dangerous arrhythmia or even cardiac arrest. When playing with these methods, I recommend monitoring blood oxygen level with a wireless pulse oximeter. They are cheap nowadays and interface with a smart phone or tablet to give you a continuous reading of blood oxygen – I discuss the limitations of these readings extensively in my previous article. Blood oxygenation, or more strictly, hemoglobin saturation, refers to what percentage of hemoglobin molecules in the arterial bloodstream are carrying O2. It is normally close to 99%, and it falls as you deprive the body of oxygen. Black out occurs around 50% in my experience.