Ok, you're probably familiar with plugins that align multi-mic setups for phase (such as multi-mic'd drums). Pi is a summing engine that, when dropped as the last plugin on every track, adjusts the phase of the audio so that maximum phase is achieved. There are clever grouping capabilities as well, such as "get all the drums aligned first, then match them with the rest of the mix." It makes a big difference on your final prints, especially when transients all hit at once, and helps avoids bass/drum phase cancellation issues.
Pretty cool stuff.