According To H.T. Part 14

If you are only using lite pressure in your reins, it should never cause your horse to toss his head or throw it sideways. If your horse does this it is because he is hurting somewhere. It is up to us to figure out where. It could be in his mouth especially horses under the age of six years. Young horses are constantly shedding and growing new teeth. Always look in the corner of his mouth for a rub place or a sore. Most of the time it will be caused by a ill fitting bit. You should check the width of your horses mouth and the bit should be about 1/2 wider, giving a little room on each side when you pull the bit up in his mouth. If you have a wider mouth piece the post will not be in the center and it will aggravate him. If you have a snaffle bit it will pull back and forth and where it breaks in the middle will be rubbing across the roof of his mouth. I do not like to ride a horse with a chain curb. Often this will cause irritation which will also cause head tossing. If your horse is in pain he will be thinking more about that than what you are trying to teach him. The less pain you cause the faster your horse will learn.