Use of whistles to relay commands in battle

[Sardaukar]

In Hellenistic phalanx, when phalanx was starting to break/rout, it could be predicted/observed when movement was seen inside phalanx according to ancient sources. Thus I am bit sceptical about rotating ranks of troops inside century, because it could be easily mistaken by other troops as unit routing.

My personal view is that rotation was unit-based (apart from replacing casualties), not within century. And in sources, when fighting in usual triplex acies, second line got embroiled in fight quite quickly. This could mean that unit rotation was provided by centuries from second line.

Interesting discussion, though. And I agree with Brent that Cornicens and Bucinare were most likely used in any movement-related commands.