Duckbill valve

A duckbill valve is a check valve, usually manufactured from rubber or synthetic elastomer, and has two or more flaps, usually shaped like the beak of a duck.

[1] It is commonly used in medical applications to prevent contamination due to backflow.

The other end, the duckbill, retains its natural flattened shape.

When a fluid is pumped through the supply line and therefore the duckbill, the flattened end opens to permit the pressurized fluid to pass.

When pressure is removed, however, the duckbill end returns to its flattened shape, preventing backflow.

Duckbill exhaust valve for Draeger twin hose regulator
A cross-section diagram of a Duckbill valve. The upper pipe depicts normal flow of fluid, while the lower pipe depicts stoppage of backflow.
Duckbill valves attached to the hood and shoulders of a Soviet "GP" professional dry suit