Masa'warty 3200... Talk To Me!
The dump vale on a 1.8 T audi/vw/skoda/seat etc engine is used When the excess boost from the turbo is no longer being used, for example when you lift off, it vents it back through the intake to keep the turbo rpm up, as its a low pressure system compared to Scoobies and stuff.
With an atmospheric valve it just dumps excess boots pressure out the system (to atmosphere hence name) so that the boost pressure doesn't stall the turbo as the boost pressure would push against the closing throttle opening hence the pressure would back up through the system and cause the turbo to slow as it would need to spin against the pressure.
Someone please correct me if i am wrong. As i usually am.