Here's what Rothwell say about the Hot Little Knob
The Hot Little Knob is a great way to get fatter, higher output sounds from a strat without resorting to battery powered electronic circuits which require heavily modifying (and often routing out) the guitar. The HLK is totally passive (no batteries required) and fits standard strats and strat clones without routing. It replaces the middle pickup tone control with a push-pull device which leaves the guitar exactly as standard with the knob in the down position (with an unchanged appearance, too) but gives a range of new sounds with the knob in the up position. The new sounds are based on a series combination of the middle and bridge pickups, but with special filtering to control the way in which the two pickups interact. With the Hot Little Knob activated the five way selector switch is still operational but gives access to bigger, fatter and higher output sounds which are better able to drive amps into distortion. All the new sounds can be controlled and fine tuned by rotating the knob. Here’s what the HLK does in each of the five pickup selector positions.
What was the bridge pickup alone is now the bridge pickup in series with the middle pickup but with the control and filtering effect available from rotating the knob. This gives a range of fatter sounds than is normally available from position 1. With the control turned up to 10 the sound is thick and dark. Turning the control down to 1 makes the sound brighter and clearer at the top end, but the weighty bottom end remains.
Same as position 1.
Similar to position 1 but with a fixed amount of low end cut to give a sound more suitable for clean rhythm playing. Rotating the knob varies the sound just as in positions 1 and 2.
Same as position 3 but with the neck pickup now in parallel with the other two pickups. This gives a warm sound similar to the neck pickup alone but with added sparkle and a slightly higher output.
Standard configuration - ie neck pickup alone.