This is the problem with fixed reins. I know alot of the main training of horses is about the front end - if you look back to old classical texts (which is supposed to be very similar to good western riding) it is about building the back up properly (lunging, long reining and groundwork including shoulder-in, sidepassing from the ground). There is so much research showing that artificially tying the head down does nothing but build the wrong muscles. I guess your horse Dingbat is just showing you how hard it is for him to do. I think we would be the same if someone strapped our heads down, after a while our muscles would feel tired, go into spasm, etc. We can say - the horse cannot.

There is a good website that explains things relatively clearly:
http://www.sustainabledressage.net/collection/false_collection.phpIf you scroll down the page she shows the effect of pulling in the neck when the rest of the horse is not ready. I understand that a horse will naturally do this, without force or gadget once it is physically ready.
At the top and bottom of the webpage she also has links linking to other topics to do with collection. I know the site is mainly about dressage it is also about good horsemanship - which is what we are all trying to aim for.