Flipper is a simple little plug that does an important job. It takes the input signal, in either mono or stereo, and inverts the polarity (a.k.a., "Phase Invert") just like the "Phase Invert" button found on most larger mixing consoles. If your mono or stereo tracks sound unusually thin it could be because of phase problems.
You can flip the phase of a wave file using wave editors but once you do you're more or less stuck with the result. Instead, INSERT Flipper into your mixer channel INSERT and try it out in real time. Flipper can also be used to purposely generate out of phase signals or as a simple "Vocal eliminator" on stereo mixes (flip the phase of one side and mix the stereo tracks together in mono - whatever was centered in the mix will disappear).