The fibers are mostly nonconductive, but with a few strands of metal woven in, as a part of the spiralling.
A strongly biased circuit gets shorted to ground by extraneous foliage, triggering arcs across gaps, as electrons rush to fill the void. Such shorts waste calories, plus add annoying ticks to the TV.
The horses, stubborn pony or whatever, will presumably wise up after testing their freedoms (perhaps a simple sniff communicates the presence of high voltage), but foliage doesn't revector itself so easily in response to the threat of shocks, isn't really the intended target of this fencing.
So after Élise stretched some new line, Les went around with the pruning shears, redefining the intended circuit.
Many farm engineers cut away all foliage to at least four feet on either side, but here they allow fencing to plunge through some smallish thickets, in keeping with the "less manicured" look, closer to wilderness perhaps, but still quite usable by the so-called civilized.
