I have read this in a book, but I can't remember where.

It occurred during the Marian reforms down to the manipular level. We know that Romans did not always fight in a tight shield wall, but left about a body gap between soldiers to be able to swing their swords and to expand the line across the field of battle and take up space. How I read the action being portrayed was that the front line troops either were killed and the next guy in line stepped up to take his place, or he killed the enemy in front of him, sidestepped (not turned around) and then backed up to the rear of his formation. This allowed the troops to rotate through and maintain stamina and all gain experience after the battle.

It's a very reasonable exercise, but portrayed incorrectly by the history channel.