My guess that the heating system has a small diameter single pipe central loop that runs from the boiler to the first apartment and then onto the next apartment and the next apartment and so on until the loop returns back to the boiler. Each apartment has two closely spaced tee's that draw the hot water from the main loop and circulates it around each apartment. The boiler's job is to keep the loop warm. Each apartment draws as much heat as it needs from the main loop. Turn off the apartment pump and no heat is removed from the central loop.
Concept is similar to the series connected current transformer driven lights used at airport runways. Each light draws what it needs and no worry about light intensity dropping due to voltage drop.
