Inside of the basin, there are 2 sets of conductivity probes to detect if water has reached a certain level. Since water is conductive, we run a low voltage current through the probes. If the circuit is complete, we know water has reached that level, if the circuit is broken, the water isn't.
The lower set of probes is used to detect when to refill the post after the horse has drunk past a certain point. The higher set of probes is used to identify when the water reaches the top of the basin so the water does not overflow.