How does a thermostatic radiator valve work?

A radiator thermostat is a modulating control that allows proportionally more flow for a greater demand, and less flow for smaller demand, thus controlling the amount of heat available to be emitted by the radiator. When the air temperature has risen to be the same as the set point temperature, the radiator thermostat will not be fully closed; it will still be partially open. This is so that the air temperature can be maintained by the flow through the radiator thermostat. If the valve were to close fully, the temperature would fall as the radiator would start to cool down.