Adjustable phase is intended to aid in blending the subs to the midrange speakers, or act as a crude "delay" element.
first, you might like to understand phase. phase can apply to any periodic waveform or sequence. eg:
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 ...
a 180deg phase shift would be:
5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 ...
but in this case there are other possible phase shifts, like 36deg:
1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 ...
notice that the phase shift doesn't seem to matter. indeed, in most cases it doesn't. but the issue arises when you want two waves to combine correctly.
with most audio systems, there is a strong filter on the front midwoofers, and on the subwoofers. so the only bandwidth that the two compete are near the crossover. the adjustable phase acts as a small delay element over this bandwidth to try to allow them to blend.
the actual adjustable phase shift circuit isn't really constant over all the audio range -- it really is just based on an "all-pass" filter. as such, you should only expect to adjust the phase for one frequency.
this should be suitable for steep crossovers as well as any fine phase shifts for single tones.