The determines whether coolant should be pumped back into the engine directly or cooled first.

The THERMOSTAT determine whether coolant should be pumped back into the engine directly or cooled first. The flow of coolant in vehicles is controlled by the thermostat which allow the engine to increase in temperature up to the acceptable level, the thermostat then has to decide whether to send the coolant back into the engine or to have it cooled first.  

I'm thinking it's the egg beater.