A mechanically driven supercharger offers exceptional response and low-rpm performance, as it does not rely on pressurization of the exhaust manifold (assuming that it is a positive-displacement design, such as a Roots-type or twin-screw, as opposed to a centrifugal supercharger, which does not provide substantial boost in the lower rpm range), but is less efficient than a turbocharger due to increased parasitic load.
The unacceptable lag time endemic to a large turbocharger is effectively neutralized when combined with a supercharger, which tends to generate substantial boost pressure much faster in response to throttle input, the end result being a lag-free power band with high torque at lower engine speeds and increased power at the upper end.
A twincharging system combines a supercharger and turbocharger in a complementary arrangement, with the intent of one compressor's advantage compensating for the other's disadvantage.
The supercharger provides near-instant manifold pressure (eliminating turbo lag, which would otherwise result when the turbocharger is not up to its operating speed).
With series twincharging, the turbocharger can be of a less expensive and more durable journal bearing variety, and the sacrifice in boost response is more than made up for by the instant-on nature of positive-displacement superchargers.
Parallel arrangements typically require the use of a bypass or diverter valve to allow one or both compressors to feed the engine optimally.
[1] Additionally, multiple companies have produced aftermarket twincharger kits for cars like the Subaru Impreza WRX, Mini Cooper S, Ford Mustang, and Toyota MR2.
Both methods involve combustion in the exhaust manifold to keep the turbocharger spinning, and the heat from this will shorten the life of the turbine greatly.
A twin-scroll turbocharger design uses two separate chambers to better harness energy from alternating exhaust gas pulses.
This also produces more exhaust gases so that the turbocharger reaches operating speed faster, providing more oxygen for combustion, and the N2O flow is reduced accordingly.