Plastic creeps away from the pressure, sliding down the threads so that even a tightened screw loosens almost instantly.

FORBES: Magazine Article

These valves use a screw type valve instead of a simple flap, and they have a pressure regulating mechanism that will vent water to the ground if the intake pressure exceeds a set level.

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Batsmen feel under pressure when they are forced to play out a string of maidens and you really feel the screw tightening all the time.

BBC: Jonathan Agnew column