You get a few types. You get that in the video that our good man corofin posted.
Then you get the type where the management retards timing so much that it backfires and aids spool on pull of.
Then you get the proper one
Its an extra combustion chamber with an injector and spark system on the inducer side of the turbo exhaust housing. Works like a miniature jet engine.
This makes boost while the car is idling