One soldier would attack, one defend. The defender was responsible for maintaining a steady distance between them, while the attacker tried to either close or extend the distance. The tactics: If the attacker backs up, the defender moves forward, maintaining distance. If the attacker moves in, defender moves to the side, maintaining distance without giving up ground and initiative.
I did this same exercise in fencing class (of course, with fencing gear and rubber tip foils, so it doesn't quite measure up to Dave's army training
. In fact I took fencing up just to improve my aikido ma'ai. I ended up dropping fencing all together, cause I just wanted to move out of the line of attack all the time, I mean what the hell is this parrying and just backing up I thought. It did help me realize or solidify the idea that ma'ai is not a linear concept, though.