The guy captured in the video uses fragments of hard ceramic material that are used for the spark plugs to break the window glass. There is a separate demonstration in a different video showing the exact procedure and the real materials used.
The ceramic fragments used here have a high density, which makes it easy to dismantle the glass window. In the video, the man is wetting these ‘probable’ ceramic bits with saliva, so they don’t produce a lot of noise when the mixture reacts with the glass. The reason why they used saliva isn’t very clear though.
The basic argument behind the use of saliva; however, can be this; Saliva can strengthen aluminum oxide to make it harder. The ceramic which is used in the spark plug is super hard than even the natural rocks. Besides, it has very sharp corners making it easy to concentrate the hit onto only a small region on the glass.
This makes it easy to scratch and even break the glass. When you use this technique to break a safety glass, the damage passes through the glass very quickly, making it release tension hence the ease of shattering.
It’s the same technique employed by glass breakers. They also have sharp hard edges which, when tapped on the glass, makes it break into tiny pieces easily. It may not work on all glasses though, but the ceramic may leave some scratches on the windscreen.
Whichever the case, don’t leave any valuable item in your car. Which the full video demonstration of the technique here.