Lenz Digital Plus LK200 Mode D'emploi page 21

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This short circuit can be avoided only through a polarity separation on both sides. A separation only
on one side, at the entry of the loop, does not suffice, because the wheels, when crossing the
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separated tracks, reconnect the tracks and will thus cause the short circuit. Therefore, a separation
is also required on the other side, at the exit of the loop.
Now, you "only" have to ensure, when crossing one or the other separation, that the polarity of the
loop is set so that the wheels cannot cause a short circuit.
To begin with, the polarity of the loop is set so that a short circuit cannot occur during entry. While
the train is in the loop, the polarity of the loop is reversed so that a short circuit can also not occur
during exit.
In conventional operation, however, this creates a problem: If the polarity is reversed while the train
is moving, the train changes its direction of motion, because the direction is dependent upon the
polarity of the track. Therefore, the polarity also needs to be reversed at the control panel in order
for the train to maintain its original direction of motion. Hence, an interruption of travel is always
necessary.
This is not the case in digital operation: Here, the direction of motion and the polarity of the track are
not dependent upon each other. Therefore, the polarity of the loop can be reversed during operation
without the train changing its direction of motion or having to be stopped.