He has a flail. When stationary the knives will drape and cause a rather firm preferred position; unlike a rotary cutter.
Great description of my difficulty mounting the flail mower! What you describe is exactly the problem. Not only do the knives drape, they tend to roll back to that position when turned and released causing the splines to be misaligned again.
I had a couple of friends help mount the mower yesterday. I have some Johnson Grass that is approaching head height (and I'm 6'-4") in places that is starting to go to seed and needed mowing asap. I didn't use the imatch or overrunning clutch. The overrunning clutch would have required shortening the mower PTO shaft for clearance. I think the overrunning clutch will work out ok without shortening the shaft when using the imatch.
I ordered a quick hitch top link adapter last night to hopefully use the imatch on my non-quick hitch compatible implements. I'll still have to dismount the tractor to hook up the top link, but that should still be easier for me to manage. I'm not going to give up! I'm too old and cantankerous to do that! Thanks, Vic