Virtually all SC applications make use of a bypass valve including all of the BBSC variations. The bypass valve is open under vacuum conditions and lets the air coming from the SC escape as opposed to being pushed up against a nearly-closed throttle plate. As soon as vacuum goes to zero, the bypass closes and the compressor is pushing air directly into the engine (no bypass).
Another very practical reason for the bypass is when the throttle is lifted under boosted conditions. Without a bypass valve, there is a shockwave as the throttle plate shuts that is arguable damaging over time to the internals of the SC. With a bypass in place, as soon as the throttle is lifted, vacuum is restored and the bypass opens providing an escape route for the air coming from the SC.
The HKS and similar bypass valves have questionable practical benefits over the more common Bosch type (plastic) units. However the HKS has a very distinctive chirp-whistle sound that tends to audibly advertise the presence of a compressor.