exhaust goes from engine into small turbine, then out of small turbine into large turbine...
air goes into large compressor and then into small compressor and into engine.
it is a multi-stage compressor setup... they two compressors share the load and are able to remain in the heart of their efficiency zone.
compression creates heat, and the larger the jump, the higher the heat. the higher the heat, the less oxygen at a given pressure (boost)
so instead of a single compressor making 50psi, you can have the big compressor make 30psi (which is a walk in the park) and then feed that pressurized air into the inlet of the other compressor which can then apply the same pressure ratio and yield more boost AND a lower charge temp. Which means a LOT more oxygen! which means more power, lower EGT's, less smoke, etc.
