The sorta of rank rotation depicted in the movie is BS, it is simply imposible as ranks would compres under presure. Maybe it would work during the battle pulse but that only happens after several minutes of intese combat when man on both sides are already exousted after a "pushing mach" even the man behind the front ranks would suffer this.
Second thing that is a bit disturbing too me, if this would happen during a "push", is that after giving a front, most likely the whole unit would colapse as it gives the oppositon a small but not neglectible "charging space".
Third, sichronization would have too be beyond modern day and only soldier with years of traning would be able too perform it without having holes and gapes in front line(not a good idea during a HtH combat).

So most likely front soldier would fall back from front ranks only after he lost his weapon or shield and how that worked in a heat of battle is more or less a mistery, but surly it was way more chaotic than depicted.



PS: ACS could u possibly put that source up in english or german my italian sucks as it is.