From Wikipedia:
A very different method of popcorn-making can still be seen on the streets of some Chinese cities today. The corn is poured into a large cast-iron canister- sometimes called a 'popcorn hammer'- that is then sealed with a heavy lid and slowly turned over a curbside fire in rotisserie fashion. When a pressure gauge on the canister 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 the popcorn explodes at once and is poured into the sack.