It lifts the air-piston off it's seat to let air flow past internally and equalise postive and negative pressure.
Don't try to unscrew it, it'll just break. I've had to drill the rivet out of one before to fix a leaking o-ring inside. But that's not common at all.
Don't try to unscrew it, it'll just break. I've had to drill the rivet out of one before to fix a leaking o-ring inside. But that's not common at all.