The Phase option allows a user dial in an exact flow pattern when using 2 or more Nero powerheads.
Your Nero pumps will have an identical flow setting. You can also think of 0 Degrees as a "sync" mode. For example, if your first Nero is at 100%, your second Nero would also be at 100%.
Your first Nero will turn on at the 0.0 second mark and turn off at the 1.0 second mark. Your second Nero will turn on at the 0.5 second mark and turn off at the 1.5 second mark. 90 Degree settings help create a very dynamic flow pattern.
The second Nero will be on the exact opposite schedule of the first. For example, if your first Nero is at 100% the second Nero would be at 1%. You can think of 180 Degrees as an "Anti-sync" mode.