A bypass valve is a one way "dump valve" that quickly vents the boost pressure remaining in the air tubes and intercooler when the throttle is closed immediately after the use of boost. The BPV is actuated by the intake manifold vacuum created when the throttle is closed. The vented air flow is sent back into the system between the Mass Airflow Meter (MAF) and the turbo. If the system is a type called "speed density" wherein no MAF is present, the BPV can be vented directly to the atmosphere. In such an installation, an air filter must be placed on the exit of the BPV as it will become an air inlet while held open by the intake manifold vacuum.
MaxG took a different and fresh approach to creating a high performance BPV. Our goal was to reach the fastest possible response and very high flow rates in a compact valve. This very fast response reduces the pressure build up in the intake tubes when the throttle is suddenly closed. This pressure "spike" is caused by the momentary high speed spinning of the turbo and is very undesirable. The higher the spike and the longer it lasts, the greater the reduction in turbo rpm results. In some cases the pressure spike will oscillate causing an "turkey gobble" or "water hammer" noise which is both unpleasant and performance robbing.
Clearly, the slower the turbine speed, the poorer the response to get back under boost when the throttle is re-opened.