Basically the fireman switch is a simple on-off SWITCH. When it is ON, it opens an override on the heater that lets the heater run. When it is OFF, then the heater is OFF.

What does a fireman's switch do?

A fireman's switch is a specialized switch that allows firefighters to quickly disconnect power from high voltage devices that may pose a danger in the event of an emergency. ... For exterior devices, the switch should be adjacent to the controls for the devices.

What does a fireman's switch do on a pool heater?

The 156T4042A Fireman Switch provides coordinated control for any pool/spa heater and filter pump. It is designed to turn OFF a gas fired pool/spa heater approximately 20 minutes before the filter pump shuts down.