CandyCritic sez, "The kernels are sealed in that thing he's spinning around. As they heat up they don't explode, because they're sealed with no air going in or out. He heats them up to a certain temperature and then he puts the sealed container with hot kernels into a bag. He then opens the container in the bag, allowing the kernels to expand, and every single kernel pops at the same time."
(Thanks, CandyCritic, via Submitterator!)