Although it's definitely not in a drill manual, I would suppose that any soldier with any semi-formal education would understand that it would be the modern day equivalent to immediate action if your caught in an ambush. If marching in column and shot at from the flank at close range - that's the command I'd give , and I'd expect my boys to respond appropriately. May sound goofy, but a lot of small unit commanders had their "stretches" when it came to drill.
That's my guess.