That's easy physics, you can try it for yourselves, all you need is a large plastic rubbish bag, a length of pipe (could be a vacuum cleaner hose if you wish), and your lungs.
First, put the bag onto the pipe and hold it in place with your hands, take a deep breath, press your mouth up against the pipe to seal it, and blow, note how little it inflates...
Second, repeat the same procedure, only hold the pipe a couple of inches from your mouth, and note how much more it inflates...
Air is drawn in as you blow down the unsealed pipe, thus you have more airflow which adds to your lung's capacity, increasing how much air will fill up the bag...
That's the basic principle as to how these air-powered vacuum cleaners work, the small amount of compressed air draws in more air which is pulled through the hose and into the bag (or in the drum in those industrial models), and the bag (or filter) traps the dirt as per normal motorised vacuum cleaners...
Simple, effective engineering, you can't beat it...
