If you want to isolate the vocals from a music track you can use this effect. You can select one of the three levels according to your needs. Light will produce less background music reduction, while aggressive will reduce more.
Note: It is impossible to isolate vocals perfectly without the original mix track. You will notice some music remains. The effect will also not work on some files which have previously encoded in a highly compressed form like mp3 (because this remove some stereo depth).