I dispute that claim. Take a Tom Thumb or any curb bit with a jointed mouthpiece You can even try it with a kimberwick that has a jointed mouthpiece. Wrap one hand around the mouthpiece. Take the other hand and pull back from the base of one shank like you’re pulling on one rein. What happens? The mouthpiece rotates down and back because the shank acts like a lever. In a horse’s mouth, it rotates down into his tongue, not up into the roof of his mouth.
About the only way to give a horse “a nice jab in the roof of his mouth” with a curb bit is to use one with a high enough port to rotate up into the hard palate.
If you want to see a bit move in a way to give a “nice jab in the roof of his mouth,” do the same experiment with a single jointed snaffle. When you pull back on the bit ring, the joint of the mouthpiece moves up, like it’s going to poke the horse in the roof of his mouth (the hard palate).
Lots of people think a single jointed snaffle pokes a horse in the roof of the mouth, but it turns out that isn’t true, either. Dr. Hilary Clayton’s work at Michigan State using X-rays and fluoroscopy showed what really happens.
And her studies have found that the action of a single-jointed snaffle is not a nutcracker effect, as has long been believed. What really happens is that, when you apply pressure to the reins, it pulls the joint away from the roof of the mouth, or hard palate, to compress the tongue against the bars of the lower jaw. Horses, it turns out, like the snaffle because they don’t like getting poked in the palate and because the tongue cushions their bars.
There are lots of things that “everybody knows” that aren’t actually correct.
Dr. Clayton has published several articles that can be accessed online. You can easily find them with a google search.
And to repeat myself, I don’t think the western Tom Thumb is a particularly good bit for several reasons. I just think singling out a Tom Thumb as a uniquely awful bit demonstrates an ignorance of bit mechanics.