A dump valve fitted to the charge pipe between turbo and throttle body. When you lift off, the closed throttle traps boost that has nowhere to go, and that pressure wave slams back into the compressor wheel, causing the flutter and bearing loading known as surge. This valve opens on that pressure reversal and vents it, so the turbo keeps spinning and boost comes back quicker on the next application. It suits any street or track turbo setup running meaningful boost, particularly gearbox-shifting cars that close the throttle often. On a mass-airflow-metered car, consider a recirculating arrangement so vented air is not metered twice.