At the rifle range, when you miss the target downrange, they wave the red disc from one side to the other instead of holding it up in a scoring position.
To me, the guy sounds like an obvious BS artist. I've worked with a few of them in the past before... they make me sick to my stomach.
Did some research, yeah, that originated back in the day when people actually worked the targets from within a ditch downrange. They would pull the target down, and a miss would get the flag waved, & general mocking. I guess some services continue to use the RSO as a spotter, here in the light infantry we do not. We have dedicated Master Gunners, that literally will lay on the back of a guy that can't shoot, look down his line of sight, and make corrections by screaming at him from about 1" away from his ear. I find this method very motivating...