How would rucking reduce concussions?
Good question, well presented.
Being totally honest it's more down to playing experience than any science, more a belief than a fact, so I'm happy for you to disagree and show me evidence to the contrary.
I was playing when rucking as it was in its older form stopped. As a flanker I tended to be the first player to the breakdown (depending on fitness levels that week). When rucking over the ball you would simply plant your foot on the ball and move it backwards. If an opposing player was in the way they would be also be moved using the same technique and as long as there was no contact with the face and neck and you didn't stamp (drive the foot forward and down) he was fair game and knew this, tending to get out the way sharpish. If the opposition got more numbers to the breakdown they could counter ruck the same way. What it normally meant was you went into the ruck with your head up focusing on the ball. Players on the wrong side got out of the way quicker and the ball came out quicker.
Now when this was banned (unsure on the exact date) when getting to a breakdown you had to focus on the opposition player and not the ball. You had to shift the player off the ball which would involve you going in a low body angle and knocking the player back with your shoulder. This looked better on the TV cameras than players being popped out the back of rucks covered in boot marks but in reality was much harder on both the defending and attacking team. When you clear a ruck out now you are effectively tackling a player without the ball using your shoulders. I found that incidents of me being clattered off a ruck when a shoulder caught my head or I misjudged a clearout and banged heads with someone increased greatly. I was in my mid 20s at this point, been playing rugby since I was 8 and had never suffered a concussion (being a kid of the 80s I had suffered with them doing other forms of mischief but not rugby) but I started to get them on at least a monthly basis and I did pack in playing rugby at that point because I was suffering from mild memory loss.
My belief that rucking will reduce concussions is totally down to personal experience but if you want a good example of what I mean watch a clear out on Richard Hibbard during the 2013 Lions test (think it was the first test) how he got up and carried on playing I will never know.