Nick is right.
As i am a canuck and because we use them all the time. In mountains where an ILS won't fit, or if it is not cost effective, Candians use a localizer backcourse (backside of an ILS)aka. back beam or a plain old localizer . I believe that I have used them in the states too. This is just a more precise non precision approach to allow lower minima and easier stepdowns (usually with a DME). Though, it is a heavy work load and not much fun.
Using a CDI (window washer), you set the front course and the needle reverse senses.
Using an HSI you set the front course and it acts like the lateral information on an ILS.
On some EFIS the display shows BC indicating a backcourse and inhibits glideslope coupling. When using a CDI or HSI, the glide slope will flop around but not give correct information. We simply ignore it. ATC does not turn off anything. You can also use a backcourse for a missed approach. Some auto pilots wil actually couple to it. Hope this helps.
[ 10 September 2001: Message edited by: tincancowboy ]