In this video, we see turbulent fast-moving smoke coming from the fire room but the smoke level in the next room is still high off the ground and not very thick. A person or crew proceeding ahead of the hose line could easily encounter this situation. The obvious thing to do is close the door to cut off oxygen to the fire and slow spread into the next room.
However, sometimes in real life, things don’t go as smoothly as you want. The house was old and the door no longer closed easily and so did not close all the way on the first try. Despite being closed almost all the way the fire continues to pour around the door. Even when the door is finally closed all the way the smoke had become thick enough to sustain the fire even without connection to the fire room.
This is a dramatic demonstration of the importance of taking a can as a search crew ahead of the hose line. With a can, this fire would be easily held in check and any extension knocked down. Without it the fire could easily ignite combustibles that would exist in a real house that had been removed for this training burn.
This is also a good reminder that smoke is fuel. All it needs to burn is the right combination of smoke density (fuel), oxygen, and heat. Once the smoke is on fire closing the door won’t stop it. It will self sustain until one of the three sides of the fire triangle goes away.