I am not such an advanced rider to get horses to carry themselves properly. But i am at a level where I can use them properly.
I have se en lots of people that set draw reins incorrectly and when using them, it goes bad.
RULE NUMBER 1
Never put them too tight. If the d reins cannot run trough the bit, then it won't work as an aid.
Like nearly all pieces of tack, draw reins have their place when used correctly. However, it's very rare that you see them being used correctly, and it's for that reason that I don't like them.
Draw reins, pessoas etc are a training aid to encourage an upside down horse with an under developed top line to drop its head and start to use that top line. The end goal is to have a horse that is capable of working through its hind end and up into the hand. I say encourage here, but what often happens is the horse is forced into on ugly, false outline by people who do not understand what "on the bit" means. Often you see them used to strap a horse's head down and in, instead of asking the horse to work themselves correctly.
Draw reins are not meant to be used for long periods of time either, nor do they have a place (in my opinion) at competitions as training should be done at home. They're meant to be used in short periods in order to help the horse to correct issues. Instead, they are often used for entire rides, and often nearly every ride. Draw reins, and anything that means a horse is working differently to what it's used to, are hard work.
The biggest point about draw reins though is that they are not necessary. What's achieved with draw reins can be done with other methods that aren't misused as much. Draw reins are just a shortcut, that many people use instead of proper schooling rather than with proper schooling. If they were banned tomorrow, I wouldn't miss them.
In the right hands I think they’re a good training tool just don’t use them all the time and be careful because they can be damaging