NAD PowerDrive ... How does it really work?

NAD has got something called PowerDrive in their amplifiers. But eventhough there is some information I am not sure how it works. They mention a SECOND high voltage supply to push in.

Here is what they say:
POWERDRIVE

To meet the diverse requirements of high current drive and high dynamic power, our patented PowerDrive amplifier circuit will build further on our reputation for amazingly effective power. By adding a second high-voltage rail to our well regulated high-current power supply, we get an "overdrive" that can nearly double the continuous power on a short term dynamic power basis. This is a further development and refinement of our renowned Power Envelope circuit, utilized by NAD in the 80's and 90's. PowerDrive differs from Power Envelope in that it offers greater amplifier stability and low impedance drive capability, resulting in less distortion when driving real speakers with real program material.