This article is from the Digital Broadcast Satellites FAQ, by Brian Trosko firstname.lastname@example.org with numerous contributions by others.
DSS and Echostar work with circular polarity. With a
single-output dish, the receiver sends a switched-voltage
signal back to the LNB in order to flip it from clockwise
polarity to counter-clockwise polarity, whichever is appropriate
for the channel you have selected. If the signal from the dish
is split to multiple receivers, there will be a conflict. This
results in one receiver being a "master," able to watch any
channel, and the other receivers being "slaves," only able
to select channels of the same polarity as the one being
received on the master unit.