The turbo spools and boosts the engine, then when you take you foot off the accelerator it closes the throttle body and all that boosted air hits it. Any excess pressure is then allowed to escape via the recirc valve and vented back into the inlet. Same as a dump valve but a dump valve lets the air escape to atmosphere rather than recirculating it back into the inlet.
With tyres ive got some yokohama paradas ready to go onto the new wheels, i got on well with them on my old white diesel fr.