It's a cheap computer cleaner balloon filled with liquid mix of propane and butane
en.m.wikipedia.org
When you push the button on such a balloon, it releases the asphyxiant gas which may be used to displace oxygen in your lungs. When your lungs are filled with a gas that has low concentration of oxygen, you quickly pass out due to hypoxia. If you pass out underwater, you can't inhale fresh air in order to recover your consciousness and then you just die because of more severe hypoxia.
The advantage of using an asphyxiant gas over hyperventilation for bypassing unpleasant symptoms of hypercapnia is that you pass out very quickly and easily without doing long and difficult tricks with breathing.