How do you get the bleeder to work on a water heated radiator? What is the principle?

Principle is to provide heat to the water from the pipeline into the homeowner's home heater, carrying a large amount of air, and some places in the heater is also easy to make these air accumulation, affecting the heater's heat, in general the air is in the water above, so the valve to release the air are in the upper part of the heater, after releasing the air in a timely manner to shut down, in order not to let more water flow out