Not sure of the origin of the term rope unit, but I can tell you that the position has evolved greatly, and our current and former coaches have played a hand in it.
Up until I think the late 70s or early 80s, there was a substitution horn that you could blow at every whistle and run a new player or unit on the field. It historically was used mainly to swap middie lines.
I am pretty sure Coach Hayes s the first coach to run on a group of 3 long pole middies at the first horn in the defensive end., as nothing in the rules prevented it. So began the era of the LSM. It became widely adopted and frankly it really slowed the game down and turned it from an athletes game to one of specialization. Control freak coaches would blow the horn at every stoppage and have specialists for offense, defense, rides, clears, etc. very different from today’s game (witch is now the best it’s ever been and the closest to the way it is meant to be played IMO).
In the early to mid 80’s they changed the rule so that you could only use the horn if the ball went out in the end line. Around the same time they limited the number of long poles to 4. Eventually they obviously did away with the horn altogether and all subs had to be on the fly through the box like it is today.
It was typical to use your 4th best defenseman as you LSM and sub them for your 3rd worst middie on defense. The other 2 middies were typically 2 way players. Around the early 90s the SSDM started to come into favor. Coaches would often use decently athletic middies that lacked stick skills and were not good enough to play in the 1st or 2nd (or 3rd) middie line and make them a SSDM. I know because I was one.
Again, I think Hayes was an innovator here as well, as I think he was one of the first coaches to have dedicated short stick middies. Their role was strictly defense and maybe gbs on faceoffs. But much like “fogos” as soon as it flipped to offense the SSDMs were expected to sprint to the box unless there was a clear fast break. But they were not expected to drive transition. This eventually lead to SSDM becoming a position that was coached, developed and recruited for, as opposed to something a middie would do to see the field (think special teams in FB. No one dreams of becoming a career special teams player besides punters and kickers, who are more like goalies or face off guys).
Brecht was one of the first coaches I’ve seen to see the advantage of having great athletes at the SSDM position (LSM has long been considered a position that requires above average athleticism). But having a “rope unit”, as it is now commonly called, that not only can play outstanding defense, but also push transition and create mismatches on offense, can be a huge advantage. You see it on most (but not all) teams now, but Brian is one of the coaches most known for using SSDMs in this way.
Any way, that’s a little bit about the origins and history of D Middies lol