And an explanation from Wikipedia:
An equally ingenious popcorn making device can still be seen on the streets of some Chinese cities today. The corn is poured into a large cast-iron cannister which is then sealed with a heavy lid and slowly turned over a curb-side fire in rotisserie fashion. When a pressure gauge on the cannister reaches a certain level, it is removed from the fire, a large canvas sack is put over the lid, and the seal is released. With a huge boom, all of popcorn explodes at once and is poured into the sack.