The tom-thumb snaffle is an ordinary and commonly used bit that comes with a range of built in bad-habit developers.
Traditionally the tom-thumb snaffle has a jointed mouthpiece that doesn't taper drastically and the bit has straight cheek wings incorporated into a small, rather than large, ring. The major advantage of the tom-thumb is its ability to stay snug against the corners of the horse's mouth with the wings preventing any chance of the horse letting the bit slide through from side to side. But it is this particular feature which also makes for the major disadvantage of the tom-thumb.
The area closest to the corners of the mouth is the area on which the two joining pieces of metal sit (where the rein ring passes through the wings). Although the wings are fixed and are not loose I have never found a tom-thumb that doesn't have a certain amount of movement or play. If you jam the fleshy part of your finger into the joint (where the horse's lips would sit) and wriggle the bit ring back and forth you will probably experience an uncomfortable pinching which would make any horse develop a variety of strategies to avoid accepting the bit - or rather, the pinching. If you can find a properly made tom-thumb that has been machined accurately, and has no lateral movement, there will be no problem but the life of this joint is limited and should be tested with the fleshy-finger-test on a regular basis.
The wings of the tom-thumb are short, flattened and are of equal length and shape top and bottom. This will give the horse the advantage of not having an uncomfortable bar pushing soft cheek tissue into the molars each time the rider tugs on an individual rein. This wing feature also prevents the bit being put on up side down because the bit is even, no matter which way around it has been buckled onto the bridle. But, because the wings are placed so close to the corners of the mouth the nutcracker action of the jointed mouthpiece is quite severe. As the rider pulls back on both reins, the bit is drawn upwards towards t he molars and the wings close directly, and instantly, against them.
This squashing procedure is not a problem with a horse that readily accepts a snaffle but any horse that doesn't will usually lift the head and open the mouth in an effort to drop the bit into the gap between the upper and lower jaw. This action gives the rider justification for using the tom-thumb because it is a bit which won't slide through an open mouth and the next usual course of action is to fit a tight dropped nose band to prevent the poor animal from continuing to take this evasive action. Like the chicken and the egg situation the horse goes on being resistant because the discomfort is still there and the rider goes on thinking they will beat the problem with an extra half-hour warm-up.
What to do? Read next month's column.