Bus air brakes are a standard feature of large diesel and electric buses, using compressed air rather than hydraulic fluid to slow a heavy vehicle safely, which is why the release produces that distinctive hiss. Online, short recordings of the sound circulate as a practical effect for filmmakers, animators, and meme makers who need a believable transit cue without recording one themselves. Its continued use comes from that instant familiarity, as almost everyone recognises the sound of a bus stopping even without seeing it.