And it looks like greasemonkey knows his stuff, I found this on the net...

"Both atmospheric and recirculating dump/blow-off valves do essentially the same thing. The function of the valve is to reduce the pressure spike that results from the sudden closing of the throttle. This spike not only slows the turbo but creates added stress on the turbine. The difference is that one reintroduces the compressed air to the intake the other wastes it to atmosphere.

With an air flow meter an atmospheric valve may create problems as the volume of air getting to the engine differs from that reported to the ECU by the air flow meter. This can result in over and under fuelling depending on the situation. A recirculating valve returns the measured air to the intake and avoids the potential problems".