How does a QEV work?

[Theegle]

So I have looked at this around here but I am still confused. How does it work?

[MrCrowley]

Spud Wiki > Pneumatic launchers > Piston valves

Or

Pneumatic Cannon Discussion (where this topic is posted) > Sticky: Piston Valves Explained Visually

QEV (Quick Exhaust Valve) = Piston valve/Diaphragm valve
Usually, we use the term QEV to describe a European or American store-bought valve whereas the French (and people on YouTube) usually use the term to describe a small homemade piston valve like this. They all work the same as a 'typical' piston valve housed in PVC fittings.

Edit: The concept is the same as a sprinkler valve; they all work due to unbalanced forces. A sprinkler valve seals the chamber whereas a QEV will usually seal the barrel.