The idea is to run two different sized turbos. Samll one for say 1800-3500 and a larger one to cover 3000+. You get low down response and top end power.

Saw a briefing paper at work this morning that said Vauxhall may produce a diesel with this kind of setup in the near future. I don't remeber the exact figures but it produced someting like 122hp/LITRE (froma diesel!!) and 500Nm of torque!

HTH.
Iain