Ok, you have the turbo spinning and pressurizing air, squeezing it into the engine. When you let off the throttle though, the turbo is still trying to compress air, but it has nowhere to go. This makes the pressure in the intake tract spike, and when it gets too high it will try to release the pressure back though the turbo (causing damage over time.) A BOV is located on a charge pipe between the throttle body and turbo, and has a vacuum hose connecting to the intake manifold. When the vacuum hose sees vacuum (when the throttle plates have been closed) the BOV vents the pressurized air to save the turbo.