The BOV is placed in between the turbo and throttle body. When you lay into the gas, it opens the butterfly plate in the throttle body and allows all the forced air into the engine. When you let off the gas, this plate slams shut, but the turbo is still spinning and pushing compressed air. This air has nowhere to go but back into the turbo which can cause turbulence and for the turbo to reverse spin which can be bad for long term life. The bov is usually a spring and and plate connected to a vacuum line (there are a few variations on the way it works) that recognizes no pressure in the manifold and uses vacuum to open the valve to allow any air in between the turbo and intake to escape. Factory turbo cars push this air back into the intake as they use metered air that can cause stuttering if it is not there. Since we have a map sensor, we just vent to the atmosphere and make cool rice noises. The problems normally are worse at high boost levels. Anything 5psi and below is not going to have major problems.
