NAD's
white paper
I'll explain it in very simple layman's terminology. There are 2 key scenarios when an amplifier's power supply (PSU) is drawn on to provide the amps, one is at high levels and the second is when low impedance speaker loads are used. As the draw from the PSU increases, especially in sudden surges, distortion arrives. This concept of NAD allows to amplifier to operate at high levels of dynamic power at low distortion levels. The key is in the design philosophy of Mr. BEE. He introduced this design initially, I think in the PE series of amplifiers (called Power Envelop technology). The concept is quite simple. Every power amplifier consists of broadly, a pre-driver, driver and output stage. The BEE design used a regulated powersupply to feed from a separate tap on the power tranny, to feed the pre-driver and driver stages of the power amplifier module while the output stage was exposed to a separate unregulated (or loosely regulated) powersupply fed by a separate tap on the same power tranny, which allowed it to draw generously and deliver what NAD called as "generous headroom". What this essentially means a NAD amplifier is able to delivery output levels in dynamic power bursts, well above their designated RMS ratings. NAD also incorporated heavier capacitor banks in their PSUs which facilitated significant current reserves for quick release during moments of heavy output load. This is also the reason why NAD amplifiers can handle a diverse array of loudspeaker loads, going down as far as 2 ohms.
I am not that great with tech explanations but hope this clarifies to some extent.