HKS is arse. The Greddy is by far better.
As the turbo pressure sensor (TPS) on the TT Supra sees increase boost, it sends an increased voltage (V) on to the ECU. When typical stock boost level is exceeded, the higher TPS V signal causes the ECU to invoke Fuel Cut; this prohibits excess boost. The BCC is simply placed in the middle of this wire from the TPS to the ECU. At peak-stock and lower boost levels, the BCC passes unaltered stock V signals on to the ECU. As stock boost levels are exceeded, the BCC inputs the too-high V signal from the TPS, but limits its output V to the ECU to peak-stock level. Fuel cut is now not invoked at over-stock boost.