The best way to think about it is to see a balanced beam on a fulcrum. Pressure goes one way and then another, in releases and charges. The triple valve controls the balance and is the fulcrum. It is a constant moving back and forth with each move of the brake valve.
One thing not mentioned so far is when a car is parked on a siding, you can turn the valve on each end and keep the air in the pipe under the car. In theory, that would allow the car to stay parked with the air in the line, but in reality there is always a leak somewhere. Thus the line left alone, will lose pressure one ounce of pressure at a time until it all hits 0. At that point there is no break pressure and the brakes release, actually before when it drops to a certain point. That is why all cars have to have the hand brakes tied down. The brakes will NEVER hold it over time.
Reading the old books on this can be very confusing, but less so than talking about how a check valve lets water of less pressure go into a boiler of higher pressure. That still gives me headaches.