One example of a backing signal being used for entry to a loco shed was at Blaenavon Low Level. National Archives document MT6-741-1 showing alterations to the station 1893/6 clearly labels the various signals.
Blaenavon was at the end of single line branch. The single line became double a bit before the station and the lines combined again after it to form a loop with platforms either side: approaching the station from the branch the left hand line was designated no. 1 and the right hand one no. 2 - the main station building was on the no. 2 line. The single line ran on towards sidings and an industrial connection.
Approaching the station, just after the line became double, was a trailing connection in the no. 1 line that crossed back over the no. 2 line and linked to the engine shed.
After this was a facing crossover, just before the platforms, so that terminating and departing trains normally used the no. 2 line.
At the branch end of the no. 1 line platform was a backing signal labelled "Backing from No. 1 line to Engine Shed".
Interestingly, at the branch end of the no. 2 line platform was the "No. 2 Line starting" signal, and bracketed on the right of this was "No. 2 Line to Engine Shed starting".
So a passenger train terminating at the station would have taken the left line at the first facing point then gone right at the facing crossover and into the no.2 line platform. The loco would have run forward out of the station onto the single line beyond then reversed back onto the no 1 line: the backing signal would clear to allow "wrong line" running to the engine shed, or far enough to allow the loco to go forward again over the facing crossover to the front of the train in the no 2 line platform.
Interestingly these signals remained long after the loco shed (and the trailing connection to it) were removed. The backing signal and the no. 2 line starting signals may be seen in the poor image number 24 on http://www.andrew.nummelin.me.uk/GWR/Bl ... efault.asp
. (This page was written before I found the plan in the National Archives.)