Newbie, coil tapping (also called coil splitting) is used for humbuckers or stacked hums....
Like what i said, its killing/grounding 1 coil, so the humbucker effectively becomes a single coil.
Thats the idea.
You can't coil tap a single coil. There's nothing to tap...or rather, to split.
Whats the relation with the vol knob?