The machine mine was connected to made a noise that sounded like a vacuum cleaner (not as loud) and there was some suction mechanism that caused the liquid to be sucked up through the tube and into a large bowl (like a big fishbowl) and you could observe the fluid so removed. The principle of physics is probably not much different than siphoning gasoline from a car- probably not much pressure is needed, once you get a backflow going. I have seen people siphon gasoline from a car with their mouths and spit out the gasoline. I know that’s gross, but it’s the same principle of physics going on with an NG Tube and it shows not much suction is needed.
Obviously nobody is going to siphon bile out of your gut by mouth, so mechanical pressure is needed to start the siphon.