The BOV does not function to 'prevent boost' it functions to release uneeded boost pressures while decelerating. The wastegates will open to keep the turbos from making boost in circumstances where it is not needed. So at idle when no boost is needed the wastegates will be open. An open wastegate allows the turbo's turbine (exhaust) wheel to be bypassed by the exhaust gasses which would otherwise spool the turbo(s). Boost control is handled exclusively by the wastegates in our trucks. How much or little boost is made is function of how much exhaust the wastegates are allowing to bypass the turbine(s). Driving conditions change and what was the perfect amount of boost pressure becomes excessive and uneeded and the BOV opens to release it. Mainly this happens when you lift off of the throttle while under boost.