http://www.enlightenedequitation.com/public/library_pelham_use.html
I have worked with countless horses that were supposed to be difficult to school, because they would not work “on the bit”. In nearly every case, I have changed the bit and ridden them in a mullen mouth Pelham, usually made of metal covered in rubber, (not Vulcanite, which is too hard and thick), or Kangaroo metal. Because the bit is mild, with no nutcracker action, and the curb chain acts on the curb groove, which contains a reflex point which, when light pressure is applied, makes the horse relax his lower jaw. It is usually possible within seconds, if not minutes to achieve this, merely by “asking” with the fingers, without any force or sawing on the mouth. His stride will become longer, his back will swing, and above all, he will learn to carry himself. I cannot understand the rationale of the “powers that be”, which mourn the loss of art in dressage, and yet perpetuate it, by enforcing the use of bits that encourage heavy hands. Regardless of the fact that I can often transform a previously wooden and resistant horse in a very short time, into one that is pliant and willing, I have nonetheless been accused of cheating, because I have used a Pelham bit, not the regulation snaffle. People always say that the horse will come behind the bit in a Pelham. This is only true if you use the bit strongly.
The horse will indeed drop behind the bit to avoid the discomfort of strong pressure on the curb groove, but when the fingers are used to achieve nothing more than the relaxation of the lower jaw, the horse will lengthen, not shorten, over his topline. I could cite countless case histories of horses whose way of going I have changed in minutes by changing to a Pelham and riding in this way.
I finally found an article that explains partially what my trainer was telling me 
And the right words for it- not unable, but sometimes resistant and lock jawed.
