Shisha Valve

The Valve across from your hose is called the purge valve. By blowing slowly into the hose you can push out the smoke that valve. A must have if you have burned the shisha or haven't blown the smoke out of the hookah for about 10 minutes and the smoke has gone stale.
The valve only gets "activated" if you blow into the hookah slowly, so it removes the stale smoke, which is less tasty, than fresh smoke.