The Flipper flips signal polarity (misnomered often as "phase"). That's it.
Why should you care, when so many plug-ins already include a polarity switch in them? Well, because so many DAWs do not include a polarity switch right on their mixers. This means that, instead of being able to try different polarities with ONE click of the mouse, you must open some random plug-in, hunt for the polarity switch, and then hit it.
The idea of the Flipper is to effectively add a polarity switch to any DAW mixer. If the Flipper is on, it's flipping polarity. If it's in Bypass, it's not.
Freeware by ReFuse Software