The variable spank circuit is a blender circuit that has a pot instead of a switch. It converts one of the tone controls, usually the lower Tone 2, into a variable control that blends the Neck pickup with the Bridge pickup in a PARALLEL connection.
Usually the Tone 1 control is then implemented as a Master Tone.
The great thing about this circuit is that full on, it’s a normal stratocaster which is important in my mind. But as you roll it off it lets you tweak the blend between the neck and bridge.
The 2nd great thing is that it’s a relatively easy modification to make.
There are a ton of variations on this circuit.
One important thing to keep in mind is that this type of arrangement works best with beefier pickups! Not all wiring options work good with all pickups. If you use this with a thin low output pickup you will be disappointed with the results. If you use it will a beefer high output pickup it gives you an awesome range of sounds.
It tends to give you the most change in sound in the lower 0-25% of the turns.