I have a French battalion acting as part of a rear guard. They are in line formation facing the enemy, who is moving forward toward them, at long range.
If I want this battalion to fall back but end up facing the enemy, then in one turn I have to Form to (Flank or) Rear, then move half a turn's movement. In the next turn they can move half movement again, then Form to Rear to face the enemy once again. Takes two turns.
No, it's a bit easier than that. When the national table says deduct half a move to move backwards, it means you can just move the unit back half a move, keeping it facing forward. See also the bottom of p.19.