Have you ever used a phaser on a bass track and ended up with a floppy bottom end wobbling all over the place? Bass Phaser can avoid that problem by separating the bass and treble frequencies, where you set the cutoff frequency. The effect is only applied to the treble portion, which is then mixed back with the bass for output.
It's not just for bass - it's flexible and full-featured. It can create a huge, wide bass sound without sacrificing punch and focus.