The sequential, two-stage charging system uses two turbochargers of different sizes with by-pass valves that direct the exhaust gas stream between the two turbines. At low engine speeds, the small turbocharger supplies boost pressure independent of engine load. At intermediate engine speeds under higher loads, both the small and large turbochargers provide boost pressure; while at high engine speeds and load, only the large turbocharger is engaged.
This is a pretty cool application of a compound turbocharged system used by Saab to improve boost response. In the video below you see the factory supplied in-dash 'turbo' boost gauge on the dash and how boost builds early in the RPM range in a very linear fashion... the car is not blistering fast, but it does have a consistent pull through all gears and does have over 300ft-lbs of torque available as low as 1700 rpms which is great for such a low displacement engine.
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