The power coming out of the wall is a sine wave. If perfect, it would have no ripples and would be a smooth 60Hz wave. The P5 and P10 work by creating a near perfect sine wave, removing the majority of noise and distortion on the line.
Paul and his clever engineers found that equipment can sound even better powered by a sine wave with an odd harmonic added. For example, a 60Hz sine wave, plus the the third-harmonic (200Hz). Multiwave is exactly this, the fundamental frequency with an added harmonic(s).
The theory is that the added harmonic gives the capacitors and other components extra time to fully top off.
The new firmware adds the ability for the end user to control how much of the harmonic(s) is added. The end user can experiment to hear what sounds best.
The downside is that multiwave can cause some transformers to hum a little. The cleaning wave does this, too. But the humming is more random sounding. Multiwave is not advised for speed controllers for turntables and may not be appreciated by some tube equipment (my thought).