Valve breathing is a calming exercise with a monotones exhale. It is as if we are releasing pressure in a controlled manner. It sounds a bit like when start to screw off, the cap on the valve of a bicycle wheel. Something like a gentle PSSSSSSSSSSSS.
While my mouth is closed and teeth slightly apart, I inhale slowly through the nose, until my lungs are full.
On the exhale I start by saying PSSSSSS slowly and gently, exhaling through the gaps between the teeth. As the air is fully expelled, the lips are gently sealed before the next inhale. Deepest relaxation point, is when the air is fully expelled.