Your video is broken. And why do I need to watch it when I already know exactly what I am talking about?
I'm bored so I'll throw in my 2 cents. Springs apply pressure pushing the brake shoes to the drums. Air pressure pushes against these springs to release the brakes. When the driver brakes he is actually venting air so the springs push the shoes into the drums. This is why trucks need to "build air" for awhile before moving off, the compressor takes time to fill the air tanks with enough pressure to release the brakes. Bonus point: This is a fail-safe system in that if the air system fails the brakes are applied to stop runaway trucks. This also has the side effect of making truck brake extremely hard and it's not uncommon for the heat buildup to cause the tires to blowout after the truck comes to a stop.
