I think you're talking about it holding boost, not revs, between shifts.
Taken from this page -
http://f80.bimmerpost.com/forums/sho...d.php?t=923203 - Two mono-scroll turbochargers
- Mitsubishi mono-scroll turbochargers
- 18.1 PSI / 1.25 BAR max peak
- Pre-tensioned in Sport and Sport+ modes (keep spinning at a high rate of speed even after throttle lift-off, for ultra quick response - more details)
- Impulse charging for optimized responsiveness
- Electric wastegate actuator