[Gandalf Parker]

Think of "attack rearmost" as being a line of sight problem. Its not, but it helps to think of it that way. The units have trouble deciding who "rearmost" refers to if they cant see them.

I like to set units such as knights or bears off to the sides (place them "low" in the image for assigning positions). Then put them on "hold and attack, rearmost". If I have enough of them then they will spread out along the side of the attacking army with some reaching into the rear. It also helps to have a group placed far on te other flank (high on the positioning square) with orders to attack rear but without the wait. They draw the enemy army up toward them giving more open room for the later rear-attackers to get past them on the other flank.

Also its handy to have lots of flyers attack archers. Archers are often placed in the rear. After the archers are dead then the flyers might move on to the commanders.

If you have an army which fatigues easily such as Ulm with heavy armor, then you might position them in the middle with hold instructions. Force the enemy to come to them and then wait for the rear attackers to do their job.