you aren't missing anything, afaik - outside scenarios all troops (except para / glider borne types) set up behind your start line.
Remember "behind the lines" is by definition not the "front" (Forward Edge of Battle Area, etc) where the action
and the game is taking place, but an all together quieter corner of the world.
Unless your campaign focuses on the deep reconnaisance / raids carried out by the SAS and other "special forces" you probably wouldnt have them in the mix of standard units.
The units themselves do exist in the OOB and are "normal infantry" (albeit with enhanced morale and experience levels) - to have them "approach a target from behind the setup line" you will need to drop them by parachute or glider - or walk them there!
