This is a great way to introduce and progress a horse that is young, or a horse that has been in a snaffle bit into a curb. I have used a Dring snaffle on my horse, Raylem, since I bought him. Then I started using a some jointed wide snaffle (I can't remember the brand right now) and my horse started giving to it great, it put more pressure on the bars in his mouth, and not just his tounge, He was between those two bits for about two years. Only recently has he gotten soft and acepted the bit bit with less and less tension, He has been bending and yeilding great to indirect pressure. So I started introducing him to a curb, just like the way explained in this article (I hadn't read this article before I had introduced my horse) After reading this article I do feel that it is a great and informative source for people who would like to start using a curb bit. Cause the results are really great! My hors will pick up a great head set right off, and is more giving in my hands and I don't have to be so strong in my cues any more.
Iv'e been working with my mare on western pleasure, and she's in a waterford bit right now. After she's fully broke I'm going to follow this and get her show ready and in a low port curb :)
Horrible advice suggesting the use of a Tom Thumb (not a snaffle bit, by the way). An Argentine snaffle is far more effective for the transition from snaffle to curb bit and does not have the confusing effect of the Tom Thumb.