The phase control does very much the same thing as the delay adjustment in an A/V receiver.
It controls the timing between the speakers and subwoofer so that their respective outputs are synchronized at the crossover frequency, at the listening position.

The easiest way to set this up is with a SPL meter and CD with 1/3 octave low frequency tones.
First set the level of the mains to 75db at the crossover (80hz for example), with the sub turned off.
Now do the same thing with the sub. ie: turn off the mains and set the subwoofer to 75db at 80hz.
The sub and speakers are now set to the same level at 80hz.
Turn on the speakers and sub, and measure the 80hz composite level from the listening position.
Room modes and acoustic interference between the drivers will either cause the composite level at 80hz to drop or rise.
Ideally it should rise, indicating that the speakers are synchronized in time as well as amplitude.

Check the level with the phase at 180 and -180; use the setting that produces the loudest 80hz tone.
If the phase control is continuously variable, as opposed to a two position switch, you can test various settings and use the one the produces the highest readings at the crossover.
It might be easier to set the phase control to the reading that produces the highest output, then use the subwoofer setup controls in your AV receiver to tweak the subwoofer "distance". (which is exactly the same thing as the phase control)

BTW, there will be some peak/null behavior below the crossover that will also be impacted by the phase setting, so it's a good idea to measure the entire range below 120hz and try to find the best compromise between precise crossover synchronization and overall low-mid bass balance.