This reverses the phase of the HI input. This is useful for phase-inverting pedals, particularly some vintage and boutique drive pedals. The "+" position is normal phase, the "-" position is inverted.
If the phase is incorrect, there will be a slight frequency scoop at the crossover center frequency. Normally, the position with more midrange is correctly phase aligned. If you can not hear a difference, go with "+", or whichever position sounds best to you.