What is a MBC?

MBC (Mechanical Boost Control)  is a VBC. - A fancy name for a simple bleeder valve. A device (air pressure regulator, aquarium valve, ice-maker valve, etc.) that bleeds off a bit of boost pressure that is supposed to be going to the wastegate control valve, thus delaying the opening of the wastegate. For more information on Manual Boost controllers on Wikipedia

Hallman MBCs tend to be well liked.