Uh, the dump valve for turbocharged engines. It does "sweesh" to bleed the excess air out of the charger when there's no throttle input. It's a functional gizmo that, when installed properly, will improve engine response on a turbo engine by a good bit.
Sadly, through Need for Speed games and Fast and Furious movies, which are both a ridiculous representation of street racing, the "psjt!" at every gearshift got insanely popular with every boy racer that thinks he knows about cars and as a result the things are very common now. Every dick wants them, every other dick falsely calls them wastegates and almost every jackass that has one has it making a ridiculous noise at low boost, like the massive muffler tips on 1.6 Civics.
