That is a definition. The thermostat rate reverses. As the set-point is reached, a thermostat that decreases or switches off its output. The thermostat that controls a typical house furnace is reverse-acting since it turns off the furnace when the set-point is reached.

What is the difference between direct acting and reverse acting thermostat?

A: Direct action means that the controller output rises if the measurement increases. Indirect (reverse) action means that the controller output drops when the measurement rises.

What is a reverse poppet thermostat?

The reverse poppet thermostat opens against the flow of coolant from the water pump. The coolant, being under water pump pressure, is used to help the reverse poppet thermostat stay closed when it is cool so as to prevent leakage. The reverse poppet thermostat valve is self-aligning and self-cleaning.