This is what the GBE BCV (Boost Control Valve) is:

A BCV is a small device, which essentially acts as a mechanical air pressure switch. A small ball bearing within the valve is held, under pressure, onto a seat by a spring. Air pressure acting on the ball causes it to lift off its seat once a preset pressure has been reached, as determined by the spring pressure on the ball – which can be adjusted of course. Once the ball has lifted off its seat air can flow through the valve. As the air pressure acting on the ball reduces below the spring pressure, the valve closes preventing air flowing through the valve. In essence no air passes through the valve until the spring or ‘cracking’ pressure has been reached.

So, here is How it works on a turbo car:

If the BCV is placed in the turbo to waste-gate actuator pipe it acts as a boost controller. The valve can be adjusted to give the desired boost pressure by simply adjusting the spring pressure. - This is done by lengthening (lowering the boost) or shortening (increasing the boost) the spring in the valve. One of the excellent characteristics of this valve is the way the boost is controlled. Due to the way the valve ‘cracks’ open once the spring pressure has been reached – no air is acting on the waste-gate actuator until this time. This means that the boost is much more aggressive and rises much more quickly than a standard turbo car. This is because normally a waste-gate actuator actually starts to open the waste-gate at very low boost levels, gradually opening wider as boost rises until it is fully open – at which point maximum boost has been reached. This has the effect of smoothing the boost all the way until maximum boost is reached. Since a BCV prevents air getting to the waste-gate until the preset boost level is reached maximum boost is reached more quickly making for a more exciting launch!