Adds variation to the tone – makes active humbucker sound thinner, closer to single coil sound, though it doesn't sound exactly like single coil or coil-split. While you add the effect, your sound gets more lows and highs, and your mid frequencies start to "travel". I rolled the knob mostly to 6-8/10, since rolling further results in way too much highs. If you have space for tone knob as well, you're gonna need it in some cases, especially when reaching 9-10/10 effect.
Although it doesn't sound exactly like a single coil, the effect is quite nice – I tried to obtain the same result using EQs in my DAW, but didn't reach the sound as natural as this expander gives. And the results I've got with it are quiet impressive – I've dialed in some beautiful surf rock and funk tones. Would surely install it on my next guitar with EMGs/Blackouts