You're partially right. The downstream sensors do switch, but not nearly as much as the upstream, and not through as great a range. The precat brick absorbs excess O2, so your mixture coming out of there is slightly rich. If the DS sensors are switching too much, that's an indication of failing cat. But you can also set lack-of-switching codes in the DS sensors, too. You should see a .3-.6 voltage range.
UPDATE: I did some testing, and with a warm engine in idle, the DS sensors should be reading about .6-.65v. If you rev the engine and back out, at least one will read low for a while, but both will slowly work their way back to the .6v range, which is slightly rich.
