With the hundreds of bits on the markets today, offering instant results, it is very difficult to decide what bit does what and unfortunately there are few people who can offer help on which one you should be using to tackle different problems.
Before going any further, it is important to note that bits vary in strength, but it is also important to note that it is not the bit that is severe but the rider's hands. When young riders are involved, it can be very difficult to find a bit that not only helps the rider stop, steer and slow down, but one that also takes care of the horse.
Terminology, such as severe or strong, are often used to describe the curb bit, but truthfully a simple rubber snaffle can be more severe than a pelham in the young, or wrong rider's hands. The terminology "strength" refers to the speed and affect a bit may have on your horse's mouth. The more places a bit can affect its action on the horse, the stronger the bit. The most important thing to remember is the comfort of your horse. If your horse is comfortable with his bit and not having to worry about it, you'll have his complete attention, which is what you want. Just imagine having something in your mouth that's bothering you and then you are being asked to concentrate...difficult isn't it.
The thickness of the mouthpiece alters the bit's action. The thinner the bit, the sharper it is, but what's more important to know is the shape of your horse's mouth. If your horse's mouth roof is low and flat or the distance between his front and back teeth is small, then chances are he will enjoy a fatter mouth piece than a thinner one.
Also, remember the number of joints makes a significant difference as well. The more joints, the more the bit can move. If your horse is always playing with his bit, perhaps considering a single joint or even a straight bar. If he holds onto the bit or his mouth is very dry, then try a French link double jointed bit. Rollers can help your tense horse relax because he has something to play with in his mouth. Thick rollers can help stop horses from leaning on the bit. The concept is the minute they try to grab onto the bit, the rollers spin. Rollers can be enough to stop your horse from tightening his jaw.
Bits are severe when they have square edges, rollers that spin from side to side or twisted mouthpieces. These bits should only be used with caution and with an experienced rider as they sit directly on the horse's tongue and in the wrong hands will do damage. Try other bits before deciding on this alternative.
Did you know that the bit rings are more important than you think. The closer your rein is to the mouthpiece the faster the affect. For example, bits with a long shank with the rein attachment at the bottom work well on the poll but can be less effective for steering.
Eggbutt and D rings gives your horse a solid, static feeling in his mouth and is helpful with a horse that plays with his bit or doesn't settle into contact. Loose rings have the opposite effect as the ring is fitted through the mouth piece allowing it to slide if your horse decides to lean or tries to get hold of the bit.
Cheek pieces help steer your horse. They also help the bit sliding through your horse's mouth and should be considered with the new rider.
Horse resistance comes in many forms, so rather than opting to use a strong bit, look at the tack, yourself and your position. Before you decide to change bits, look at your horse's noseband and bridle. An ill-fitting bridle may present issues that resemble bit problems.
Remember why you're schooling/riding your horse...it's all about pushing your horse forward to a steady, relaxed contact. It's not about pulling his head in, pulling back on a stronger bit or using draw reins. Your horse will go "on the bit" if you push him into it!
Good luck and enjoy your riding!