It’s true.
I have a zero-gallon/flush toilet! The air in my basement (collected by my 5-gallon dehumidifier) flushes my toilet.
The 5-gallon reservoir in my dehumidifier fills up daily in the summer (I’m in humid St Louis, MO). Then, gravity lends a hand. I pour the 5 gallons into the toilet to flush the “yellow” that we’ve let “mellow” in the first floor bathroom, at the end of the day (for all you cringers out there, it doesn’t start to smell until day 2 or 3).

#2 is done upstairs and flushed in a normal fashion each time–we’re not barbarians:)

Now, I know that this works (and that it saves water) but I don’t know how it works. For that I looked up an old yahoo answers post which explains the process (for those who want to know).

Source: GO Media - Written by Joe Mohr - Image credit: Wikimedia Commons GNU Free Documentation License