If there is problems with crokedness then it is imperitive that you school in a snaffle as the horse MUST go forwards first in rhythm, then comes suppleness developed through systematic work in the school, circles, loops and serpentines for lateral suppleness and transitions between and within paces for longitudinal suppleness. When the horse is supple he is more able to maintain an even contact, neither raising his head above the bit, becoming overbent or leaning on the bit.
The double will mask many of these symptoms and will often stifle forward movement - the horse feels improved but often is not working throough.
The curb for a time will encourage the horse to bring it's nose in as the chain acting as a fulcrum tightens around the jaw, and the leverage generates poll pressure. What we really want is for the horse to stretch it's neck over the nose - imagine pushing the ears forward away from you rather than bringing the nose in.
I've been working the last 6 months on getting my very high headed arabX mare to stretch down. She had absolutely NO Rhythm when I first rescued her, could not halt squarely with out taking three to four funny steps, totally untrusting of any bit/leg pressure, over flexes at the pole, and very anxious.
But now I can honestly say that she WANTS to stretch down -- It feels good to her! I do serpentines, other exercises and just now started leg yielding. Teaching a horse to stretch benefits the horse so much and it is a SHAME more people do not understand its importance.
I am slowly asking my mare to bring her front end up (after doing the above exercises for about 10-15 min) and gain a little more collection, but then I release her and let her stretch. And I know she is understanding what I am asking her because she does not over flex half as much as she used to
Oh, and for bits, I alternate between a baucher bit and a french-link (I'm trying to see what she feels more comfortable in) She does not chew as much with the baucher so maybe she likes it more?
I do not believe in draw reins at all or putting more metal in the horse's mouth. Unfortunately, I have a friend who owned a beautiful 17H OTTB and she used a lot of hardware on him:
I watched her ride him one day and it was incredibly frustrating because her position was falling forward, her arms/hands were strait and very low and never let the horse stretch. She tried keeping him locked in frame but his back was completely hallowed and it looked like he was prancing on his toes... :(
She loved him very much but she was doing everything wrong . . .