I used one with my horse. Do not recommend it.. my trainer had me in it and I didn't realize what it was doing.
It has taught my horse that without it he can flip his head, he never used to do that. He leans on my hands, he resists contact.
He's slowly working on relaxing more but I can tell you prior to the fork we were getting somewhere. After the fork.. he has gotten worse and less willing.
The english equivalent is a running martingale.
In my eyes the ONLY way to teach a young horse/old horse/retraining horse "to collect" is to ride them correctly and over time they begin to develop those muscles on their own. Trust me.
I hate that phrase, people just don't understand. Even I don't understand it, but if I knew what I know now.. back then, never would have used a training fork on my horse.
"Strength is the ability to use a muscle without tension"