Here's what Rothwell say about the Neck Adder
The Neck Adder is a rotary control which will blend in the neck pickup with the bridge pickup, middle pickup and bridge/middle pickups on strat type guitars.
This gives you the missing pickup combinations on strat type guitars, ie neck + bridge and all three pickups together.
Using the neck and bridge pickups together gives a sound similar to a telecaster, and using all three pickups together gives a bigger sound than is normally available on a strat. However, since the Neck Adder is a rotary control and not a switch, as much or as little of the neck pickup sound can be added to the other pickups as is required.
The effect of adding a small amount of the neck pickup to the bridge pickup, for example, is that the sound has a little more warmth and fullness - perfect for taming slightly shrill bridge pickups.
The Neck Adder replaces one of the tone controls (whichever you chose, but the middle pu’s tone control is usually the easiest) and will blend in the neck pickup when the 5 way selector switch is set to the bridge pickup, the middle pickup or the bridge + middle pickups.
When the other two switch positions are used the Neck Adder will have no effect (because the neck pickup is always fully on in those positions).
Turning the control fully anti-clockwise leaves the guitar exactly as standard (except for the loss of whichever tone control you chose to replace.)
No ordinary pot
The Neck Adder isn’t just any ordinary pot. For this concept to work correctly and smoothly a special, custom made pot is required. The Neck Adder is a reverse log taper pot which also goes open circuit at the bottom end of its travel, effectively acting as a switch as well as a pot. This ensures that the pickup selections work exactly as standard when the knob is turned down to zero and the neck pickup is blended in smoothly as the control is rotated. No ordinary pot can adequately replicate what the Neck Adder does.