How did the Romans swap units around mid-battle?

Great Courses Plus free month offer: http://ow.ly/gAEv308jLrJ How did the Romans replace hard-pressed units with fresh troops during battles? The theories on this are legion. Course featured, "Greek 101: learning an ancient language" taught by Hans-Friedrich Mueller, Ph.D. of Union College, Schenectady, New York. "The Great Courses Plus is currently available to watch through a web browser to almost anyone in the world and optimized for the US market. The Great Courses Plus is currently working to both optimize the product globally and accept credit card payments globally." The spelling of 'optimise' in that statement was optimised for readers in the USA. Here I exercise my right to speculate about how Romans swapped maniples from the second line (principes) with units of the first line (hastati) during a hard-fought battle. I am confident that no one knows how it was done, so we can all give it a go. Support me on Patreon:   / lindybeige   Pre-order my book on Hannibal and the Second Punic War: http://www.InSearchofHannibal.com Buy the music - the music played at the end of my videos is now available here: https://lindybeige.bandcamp.com/track... More weapons and armour videos here:    • Weapons and armour   Lindybeige: a channel of archaeology, ancient and medieval warfare, rants, swing dance, travelogues, evolution, and whatever else occurs to me to make. ▼ Follow me... Twitter:   / lindybeige   I may have some drivel to contribute to the Twittersphere, plus you get notice of uploads. Facebook:  / lindybeige   (it's a 'page' and now seems to be working). Google+: "google.com/+lindybeige" website: www.LloydianAspects.co.uk    / user  "Lindybeige"