New DOE rules changes home hydronics systems, Solving some common system problems, controlling air, Controls and devices for heating boilers, Trouble shooting domestic steam heating systems, The high cost of dirty water in domestic boilers. By regulating the amount of air released, more or less steam is permitted to enter the radiator. The result is more or less heat emitted from the radiator. When steam heats the valve control inside the vent, the device closes the hole, preventing the loss of steam. When the radiator cools after the thermostat shuts the burner off, the vent opens to permit air back into the system, preventing collapse from atmospheric pressure.
Steam heat is a matter of timing, the vents on the side of the radiator provide that timing.
You might swap some of the upstairs vents with the downstairs vents to see if that might help. It is the hole in the vent that permits air to leave the radiator at a given rate. The faster the air leaves, the faster the radiator will get steam inside. Vents come in different venting sizes to adjust the timing - some are adjustable, others are automatic. You will need to find vents with A,B,C,D,E, or 1,2,3,4,5 sizes to make the venting time longer in the upstairs rooms or slower in the downstairs rooms, or a combination until they all heat evenly.