RST Engineering wrote:

Not only is it remarkable selectivity, but crossmod, intermod, and all the
rest of the *mods combined with the antennas less than 5 feet apart, a 10
watt transmitter and a 1 microvolt receiver. One might be tempted to call
it a miracle.

I remember some of this vaguely from when I was active with amateur
radio but all that info is long gone. How does it work then?