Here's what Rothwell say
The ACT is an advanced coil-tap which gives the clarity and sparkle that conventional coil-tapping does but maintains the full bass and volume that conventional coil-tapping does not.
It is a well-known fact among guitarists that coil-tapped humbuckers give, at best, merely a passable approximation to a single-coil sound.
The bass is usually weak and the volume drop is considerable, and this means that coil-taps are often a rarely used feature when fitted to humbuckers. However, this doesn’t have to be the case. The ACT eefectively operates as a frequency selective coil-tap, giving the top-end clarity of a single coil pickup whilst maintaining the bottom end fullness required for a satisfying sound.
The circuit is derived from the Cool Little Knob but unlike the Cool Little knob has the advantage of being able to operate on two pickups simultaneously whilst leaving the pot available to be wired as a volume control or as a conventional tone control. Whereas the Cool Little Knob gives a continuously variable range of sounds from a humbucking pickup, the ACT gives a fixed “single-coil” sound - but the exact nature of the sound can be pre-set from 4 different options using the miniature switches on the circuit board. This feature retains some of the Cool Little Knob’s versatility whilst allowing the ACT to be simpler in operation.
The ACT comes complete with easy to follow fitting instructions and is compatible with all humbuckers which can be coil-tapped. Now there’s no need to endure the thin and weak sound of conventional coil taps when it is so easy to replace them with an ACT for a much more satisfying “single-coil” sound.