What is safety device / FFD (Flame failure device) in hobs?

Safety device / FFD is used in gas hobs to prevent the accidental gas leakage. In case if while cooking the flame goes off then safety device / FFD immediately cuts off the gas supply to the burner. This is very safe because such incidences happens and when you are not in the kitchen then gas will be leaking continuously unless someone turns back the knob at off position.