Dairyground, if your bum is on the seat and the mask is on your face then the pin is pulled and the candle's burning. The lanyard won't reach a face that's sat on a seat.
And a demonstration of the force required to pull the mask? My two year old daughter would have the pin out without even trying! As any engineer will tell you, just looking at an oxy generator pin is enough to have the bloody thing fall out!