What is this radiator vent used for and how is it used?
According to the manufacturer, Heat-Timer Corp. The item pictured is a "Varivalve" air vent.


  "The function of the radiator air vent is to provide a means for air
  to be driven out of the radiator as steam enters. At a given pressure
  the fill time of a radiator varies proportional to its heat capacity
  and inversely proportional to the venting capacity of the radiator air
  vent. It is possible to control the speed at which a radiator will
  fill with steam by installing a radiator air vent with a certain
  venting capacity. Used in this manner, radiator air vents are helpful
  in evening out the heat distribution in a building."


The pin is an adjusting lever that controls how fast the air is released from the radiator which, in turn, affects how fast the radiator fills with steam. By setting the pins correctly you can prevent all the steam (heat) from running to one end of a room or building.

I don't have any first-hand experience with these specific devices, but here's a link to the manufacturer's instructions for the product.