The proper way to put a horse on the bit is to push it to the bit with your legs, keeping a light, but steady feel so that the horse has a comfortable place to put its mouth. If the animal responds to the leg aid by pulling on the reins, then you can use half-halts to lighten the horse’s forehand. (See How to Perform a Half-Halt.)
The proper half-halt requires the hands to close together simultaneously, so that equal pressure is created on both sides of the horse’s mouth. It is incorrect for the hands to wring the horse’s head from side to side. In fact, the wringing motion will prevent the horse from being able to find the bit, for the bit must be stationary for the animal to establish steady, but light, contact with it.
The key to putting a horse on the bit and keeping it there is the leg. As Bertalan de Nemethy, former coach of the U.S. Show Jumping Team, used to say, “Put two-thirds of the horse’s body in front of your legs.” What he meant was that the rider’s legs should create impulsion in the horse’s rear end, so that the hocks become engaged and the part of the horse’s spine just behind the saddle rises slightly, making the rider feel that the horse’s momentum is in front of his legs, instead of behind them. In my own experience, when the horse is correctly on the bit, I feel as though my legs are holding the horse’s entrails just in front of the girth, and that if I let off my supporting leg pressure, the entrails will spill backwards, behind my legs. This feeling is created because the engagement of the hocks–which makes it possible for the weight of the horse’s forehand to be supported by the animal’s hind legs–coupled with half-halts, which shift the horse’s center of gravity back slightly, result in the horse’s center of gravity being somewhere around the area of the withers (the same as at a standstill), instead of considerably farther forward, as the horse’s center of gravity would normally be while moving. (Note: Unchecked, the horse’s center of gravity shifts farther forward incrementally as the pace increases.)
The lightness on the bit that you see when watching very good riders is not an illusion, but simply a reflection of the rider’s ability to push his horse to the bit, while keeping a light, but steady, feel through the use of half-halts. It is true that some horses are easier than others to put on the bit, but barring conformational restrictions (such as a very short, thick neck or a ewe neck), every horse should be able to be put in a medium frame.
Of course, it is more difficult if the horse has never before been ridden on the bit. In this case, the muscles in the horse’s neck may be less flexible than in a horse that has been trained properly, and, due to discomfort from trying this new position of the neck, the horse may object to your hands. It is important when training any horse–young or old–to give frequent breaks during flatwork, so that the horse doesn’t become uncomfortable and fight you.