The Digitrax AR1 is a reversing module. It is used to automatically reverse the rail phasing on trackwork such as wyes, turntables, balloon tracks, or reverse loops. The AR1 uses a mechanical relay to accomplish this task, with the contacts wired as a DPDT (double pole, double throw) switch.
The Digital Command Control signal on the track does not have polarity. The signal has two phases: On or Off. A short circuit occurs when Rail A connects to Rail B, as the current can easily flow from the energized rail (ON, or logical one) to the grounded one (OFF, logical zero). The signal on the rails flips from on to off rapidly, with Rail A always being in the opposite state of Rail B.
Example of a Reverse Loop
When the track folds back on itself, rail A will meet rail B at the turnout. Both rails are gapped at points entering and exiting the loop. The reversing module supplies the DCC signal to the loop, taken from the track before the turnout. When the locomotives wheels bridge the gap, a short occurs when the DCC signal's phase across the gap (Rail A1 does not match Rail A2) is incorrect. The AR1 quickly flips the phase on that section of track to match that of the track section the locomotive is entering.
See reversing sections for more info.