The bit inside the 9 volt connector isn't really logic. The flip-flops and Schmitt trigger gates are just to generate square waves at 3 different frequencies. T

The bit inside the 9 volt connector isn't really logic. The flip-flops and Schmitt trigger gates are just to generate square waves at 3 different frequencies. There's also a small power supply to drop the 9V down to 6.2V for the input of that, some comparators, and a bit of filtering. The LEDs contain LC filters, tuned such that they'll light (visibly) when the appropriate frequency is applied. Touching the wire in different places creates a different parasitic load, which is detected by one of the comparators. That comparator then toggles the clock generator for the appropriate frequency, turning that LED off or on. As with all of his circuits it's a brilliantly simple idea implemented in a ridiculously constrained space.