I don't have my books on hand, so this comment will defer to the next one which can cite specific sources. But what I can say from researching this topic is that there is actually no specific evidence that legionaries in a maniple would cycle through ranks during a battle. This notion appears to have been formed by later historians who thought it the most logical reason for the ranks behind the front line. From ancient sources however, the ranks behind the front line may have simply served the purpose of pushing the front ranks forward and keeping those fighting in the front from fleeing backwards. While we typically think of the Roman army as a highly disciplined, mechanical, fighting force, multiple sources (Josephus, Tacitus, etc.) speak to the contrary. The Roman battle line may have been more like a rugby scrimmage than what is depicted in the clip above.

Maurice of Nassau, an important Dutch general of the late 16th century, claimed to base his system of drill for musketeers on the Tactics of Aelian; in Maurice's system, musketeers in the front rank would fire and retire to the rear rank to reload, with the men in the second rank stepping forward to become the new front rank, before themselves retiring to the rear rank to reload. He claimed to base this system off an alleged Roman practice of the different ranks of the century throwing their javelins in turn, but I cannot find any such description in my copy.

Some caveats apply when it comes to applying this system to the ancient Romans. The evolutions Maurice of Nassau describes seem most similar to the Macedonian method of countermarching among those detailed in Aelian's manual, which are a way of changing a unit's facing rather than replacing men in the front rank, and would typically end with the unit occupying adjacent ground rather than the same ground.