Fletchy-Muncher simulates loud volumes playback of the spectrum according to the Fletcher-Munson curve.
Put it last in your chain and if it sound harsh or too bright at the Default position you have to go back to the mix or master and change either EQ, MB compressoion, De-esser or other tools or methods to get rid of the problem. DON'T OVERDO IT, IT WILL SOUND DULL ON LESS LOUD LEVELS.
In Flat position the plugin does absolutely nothing, when you raise the knob to the Default position you will hear the difference between about 60-70dB to 100dB, wich is pretty much the gain increase from a moderate volume in a little studio to a loudest volume from a club PA. Remember to bypass or remove it before bounce your new mix/master.