How do green gas airsoft guns work?

I know green gas/propane is flammable, but i am pretty sure green gas guns don't work, by igniting propane every single shot. so is it just preasurized or something, how do they work. I don't have a green gas gun, but i do have a a pvc propane rocket launcher that i built. Although that works by making a spark and igniting th propane inside the chamber. But it doesn't seem like green gas/propane would work like co2 does for co2 airsoft guns, but i am guessing it does.