It is a metal disk that mounts on the evaporator coils. It closes the heater circuit at 25 f so that the heater can warm up the coils. When the temp reaches 50 f that is warm enough for the heater to melt frost without disturbing the freezer food. If the heater were to remain on for 21 minutes of defrosting then that would not be pretty. So the bi-metal kills the heater.