Particularly useful for adding stereo width to mono signals, this is a EQ based pseudo stereo effect that doesn't mess with the tonal quality too much and remains mono compatible (as long both L&R signals are summed).
The effect is loosely based on the same principle of the 'Stereo Spread' effect found in Logic Pro X: https://documentation.apple.com/en/logicstudio/effects/index.html#chapter=7%26section=3%26tasks=true