I am pretty sure that in Fillys' case it is a feeling that she is loosing dominance that is the issue. She was hand reared appearently, and as a young foal "allowed to wander around the yard like a dog". Thus she was taught very little respect by other horses. She was never out with them.
We have come a long long way in our time together. If you read my blog you would find that my wife would wear a body protector when playing online with her at the start, and the difficulty level of the day could be measured by pints of the beer in the pub afterwards for me :) .
The hind quarter yield problem is just a last vestigial part of that extreme behaviour. Folks at the yard who are most definitely NOT natural horsemen and have been very anti our practising it are even coming up to us and saying what an amazing change they have noticed in Filly since we managed to buy her and therefore have complete control over her handling.
I merely mentioned the current yield problem as an illustration of the topic. I know we will overcome it, it will just take time patience and understanding of her point of view to achieve it. Over the process of training her I have found that the ideas I put forward in this post have helped me emotionally to do what is required to make progress. The idea that I am doing it "for Filly and with Filly" has been important to keep emotional control in difficult situations.
One of my instructors, who has just come back for working in Pat Parellis barn with Pat himself, says that Filly is in the top 3 most challenging horses he has ever met and he has known her since I started with her. He has also said that he thinks Filly and I are perfectly matched. Not sure what to make of that :) !!