So I'm assuming that you mean someone sitting on another person putting pressure on their neck via their knee.
I find this to be interesting timing for this question, because a few days ago a man was killed in Minnesota after a police officer sat on his neck using his knee for more than 8 minutes. So that brings us to our first example:
Because it's hard to pinpoint exactly what martial arts police train in, I'm calling it police training. Usually (at least in the US) it consists of elements from Krav Maga, Hapkido, Judo, and Brazilian Jiu Jitsu. It's not common for a police officer to do this, although it does happen occasionally.
I have a little bit of experience with Krav Maga, enough to recognize this as a possible option. The reason is that Krav Maga is taught specifically for self defense and combat, so your opponents' well being is not usually much of an issue. When I've trained, we practice groin strikes, eye jabs, and other very dangerous techniques.
where would you find a training partner to let you practice such a deadly move?
Well, in Krav Maga we don't practice the techniques with 100% intention, which is why you should be able to practice this on a Krav Maga partner. My guess is that you would get your opponent to the ground and set your knee on their neck, but not apply pressure. However, I'm not sure that this would be used in KM because it doesn't really give a tactical advantage. It leaves your stomach, face, and groin open for strikes. And something like that would most likely not be taught.
If you want to practice this really badly, then attend a Krav Maga class and try it out.
In conclusion, my best guess would be that it's some mixture of various arts, along with just "regular fighting" techniques (or in this case, not techniques).