SnipeYa:
Something like a transistor that lets current flow without input and stops flow whilst receiving.
That would be a depletion-mode FET - not sure its easy to find one rated for significant power
and low on-resistance though, tend to be small-signal components, but some do exist.
Not sure about the practicalities of a depletion mode device switching a through-signal (as opposed
to just switching to ground)