well from wat i have studied, latest twin charge system of VW supposingly work like this:
1) when engine speed is low, supercharger react at 1st by the pull from crankshaft and start to deliver more air, hence increase torque and hp in low rpm.
2) when engine speed is at mid range, when the turbocharger is getting enough pressure, turbocharger will start to deliver air into the engine along with the supercharger. hence providing a high boost at the same time.
3) the supercharger is attached to a electronic clutch which receive signals from ECU. when the engine speed is at high end, we know that supercharger will waste a lot of energy from the engine due to the mechanical connection btw them, mostly lose about 30% of energy. at this time ECU will disengage the clutch and hence the supercharger wont steal any power from the engine now. while at this high end speed, engine is providing sufficient exhaust pressure to the turbocharger to turn even higher speed and providing enough boost while not losing the power like the supercharger.
in this twincharging system, turbo lag can be reduced and the high boost can be obtain. torque and hp is great as well. the fuel consumption for the VW 1.4 TFSI engine is really good while giving a marvelous power.
Heard that VW is getting a VGT for this twincharging system as well.
sorry if i had any wrong info.