Steve (itsmeagain) will give you as long a description as you want but briefly, air is allowed into the engine (in cossie terms using a modified idle speed valve to allow more flow than usual) and fuelled accordingly but with very retarded ignition - this puts enough exhaust gas through the turbine to spin it up and produce boost, but the engine speed remains low due to the over retarded ignition timing.. which in turn makes all the noise since the exhaust valves are opening while the burn is taking place. This is also why there is a lot of thermal strain put on exhaust valves and turbine wheel/shaft.