Originally Posted by MDORPHN
As I understand it, having the M button on closes the wastegates, thereby decreasing spool time and speeding the transition from partial to full throttle.
When the M button is off, the wastegates are open which reduces backpressure/pumping losses and increases efficiency/fuel mileage.
That's probably the reason why the exhaust pops like crazy when shifting from 1st to 2nd in normal mode, especialy when the engine is cold.
"... on the smiles-per-mile charts, the 1-series M coupe can't be beat. It is a challenging piece, a triple-X-rated driver's machine for semi-professionals. Its handling balance provides an open invitation to indulge in latent oversteer, and provides an opportunity to separate the men from the boys. It is an absolute hoot on the track and an addictive plaything on your favorite twisty back road. "