When the accelerator is released
When the throttle butterfly closes, a vacuum is quickly created in the intake manifold due to combustion while the pressure before the throttle butterfly remains the same.
To prevent the pressure in the inlet manifold increasing too quickly, which will cause jerking, the excess pressure before the throttle butterfly valve must be released when the butterfly reopens.
Because of the vacuum in the intake manifold, a vacuum is also created on the spring side in the bypass control valve. The vacuum causes the valve to open and the pressure from the pressure pipe and throttle body escapes into the intake hose.