I seriously don't think a rehearsed chorus line of actors represents adequate proof of something that isn't documented in Roman sources.
This is not a proof I'm talking about here, but a mere indicator of "Could it be possible to train average people to do that kind of system.
I notice that Brady, like any good officer of the 40's, stresses parade ground precision. I do have objections to this. Firstly, by what mechanism did the front rank realise they were all sufficiently tired and weakened? They were facing the enemy. They were busy in a sword fight. As for the centurion, he was likely in the front rank along with them, and there's no guarantee he was able to observe and command objectively.
Secondly, Brady may well be assuming a pyramid commnd structure as he was used to. There is no direct evidence I know of that the Romans used such a system, and the structure of the legion suggests otherwise. Although the Romans stressed individual initiative, it's clear from the sources that there was little of it in practice, and indeed, Roman soldiers who did take the initiative were as likely to be laughed at by their mates.
Thirdly, Brady was trained in a different methodology of fighting and his assumptions about how the battle line begaves is a little fanciful. He does not metion how this line relief system dealt with dead and injured legionaries obstructing the process, nor how it coped with rough ground.
Fourthly, if, as Brady hints at himself, the relief of the fighting line was done ad hoc, and not by general order, this does not represent a practised drill but simply common sense if you want to present a durable front against the enemy.
If Brady was studying battlefield tactics from Caesars time, does he quote where he got this information? Why is Brady regarded as such an informed source?
I'm not regarding Brady as "Such an informed source. I precised clearly that it was one of the reachable source I could find and I that wanted to go see HIS references. (Tracking down primary source through second and third who used it) I'm just exposing you my overall process in a kind of "Live" way.
Unfortunately, I'm in a bag end here: The person who put the text online didn't put the bibliography. I'll either have to give up that hint track, or try to find the book in real itself so I'll get the bibliography and the end of page notes.