Apocalypse by Harry Lorayne Volume 17, Number 2 February 1994 Page 2317 et seq.
Personal Comment: Needlessly complicated, that's for sure. Check out The version by Jim Surprise and Roberto Giobbi. Much better. You can also work in the little extra that is present in the above routine to get a much cleaner version. Personally I don't think the initial switch is that deceptive, even if executed properly. The ATFUS is the way to go in my humble opinion.