The oxypilot is carefully factory set so the flame just impinges on the tip of the thermocouple. If the oxygen content of the available air reduces (flue gases spill back into the roomspace) the flame lifts off the tube seeking oxygen and in so doing ceases to contact the thermocouple. The thermocouple cools and the gas valve closed.
The assembly must be kept clean as the slightest obstruction will prevent the thermocouple heating during the initial light up of the appliance.