The thing is, if you flex them a ton, and you aren't working at all with the "feel" of how they respond to the rein, but just yank their head around, you end up with a horse that's kind of broken in half. Some hroses get so worried about that rein pulling them around that when the rider takes up a contact, the horse dodges behind it , or to the side of it, but certainly doesn't "meet" it honestly. And then it's a bit like a semi truck that jack knifes; the front end is bent over while the back end just plows forward. You certainly cannot do any dressage on a horse like that. There is no way you can ride him back to front if every time you take up the front (meaning contact on the mouth), the horse just kind of "mushes" off of it.
Yeah, it's a "mushy' sort of feel that develops. Horses can use this in a very evasive manner. Especially if you one rein stop the horse by just hauling his head over to the side, but he is just standing there leanihis front legs, holding his head to the side, but his eyes and ears still indicationg that he is thinking "I am going forward again as soon as this jerk lets loose of my head". They will literally fall forward as soon as you release the rein.
I don't do any full one rein stop without asking for the horse to come all the way through and step under with the hind and disengage. Then, I won't let them walk forward simply by the loosening of the rein. The MUST stand, on a loose rein, until I ask them to go forward.
Easier said than done. This can take a long time of allowing them to make the mistake of walking off, taking up one rein. Now here's where the depth that you go into the ORS may vary.
If the horse walks off and I pick up the rein and he stops by just a lifting of the rein, and he comes off the rein (doesnt' lean on it with his head), I don't disengage the hind. If when I lift the rein and make contact, he braces against it and tries to go forward with his head bent to the side, I go "deeper" , meaning lift the rein higher, turn my body a bit to lift my inside hip off of him a bit , and encourage him to think back into his hind end. If he softens and stops, then I dont' disenage. But if he still braces and resists, I go into the hind end, ask for a full disengagement, drop the rein on his neck and test and see if he'll stand. If he does, wonderful! If not, back to the cycle just described.