Just in case any of you wanted to know, and I dug this up after assisting with an anti-skid problem.
The self test switch sends 26VAC 400Hz to two of the 4 identical circuit cards in the control box, either FWD or AFT. The 400Hz voltage is knocked down and sent thru the transducer coils back to the appropriate circuit card, basically simulating wheel rotation. Releasing the switch causes the loss of the 400Hz rotation signal, causing the system to think the brakes locked up. This send a pulse to the valve circuit and test light circuit, making the light flash.
I share this as nothing in any of the C130 books we have state that the transducer coils are part of the test.