Phase pedals operate by taking a copy of the guitar signal out of phase, then reversing.
it's all signal orientation. if the signals are all + - + - they're in phase.
once the signals have a + - - + relation they're out of phase,
it's just like having the red and negative wires on your home speakers wired "out of phase" by having one speaker have it's polarity opposite the other.
although perfectly out of phase their is no resulting signal. so for a pedal design you'd want to end up with at least a two knob pedal. (phase relation, and make up gain) so the circuit would go;
you could even have a mix knob if you wanted to.