Think of the legs and reins as a channel through which the horse is driven forward. Each side of the rein and leg, in conjunction, contain the respective sides of the horse. Remember that you must have continuous contact with both the legs and hands. Though subtle. With the legs and boot aids a rider, must earn her/his spurs. With the hands contact must be constant but not overactive. Think of the reins as a piece of thread that must keep in contact, but must not break the thin bond. A bit of an over-reach, but something that really helps once embedded in one's brain. If the rider loses contact with either it leaves a hole in which the horse will probably attempt to escape. This will always result in crookedness and poor control by the rider.
Note: Remember that both reins stablize the neck into the withers.
There are plenty of other ways the horse can be "out of frame." One of the first results of overactivity with the reins and hands is that the horse on the horizontal breaks his neck out of frame. This means the neck and spine are no longer in a continous frame overall. The horse escapes the rider, by coming behind the bit, shortening his neck. Usually a result of bullying the horse into so-called collection it can be corrected by less active hands and stronger aids leading to implusion.
An over-bent neck is usually a result of the horse trying to escape the rider, falling against the outside shoulder or sometimes called running out. The horse's neck will be broken out of frame just in front of the withers. The cure is to straighten the animal using a more appropriate outside rein. It is usually caused by incorrectly using the outside rein sending the horse mixed signals and allowing him an out.
The most common is probably the mouth opening or crossing. Once it becomes a habit it will cost you points eventually. The flashband is the obvious cure, but proper use of the hands will cure it as well. Sticking out the tongue is another problem in this vein. WATCH YOUR HANDS. It's usually caused by either forceful or even Dead hands.
Another common fault is when the horse will tip his head sideways to some extent. Always, again, a problem that begins with the rider's hands. Bullying, lack of implusion or riding with a predominate hand.
Remember that the most important function of the rider, in order to put the horse into frame, is her/his seat. When the rider sits down into the seat with her/his body in alignment it holds the horse in front of the rider. That influences the presence of the hind. Remember the hindquarters detirmines implusion and will keep the horse from falling aprt in frame. Again remember if you have control of the hindquarter the whole horse follows.
As long as the rider stays 'on the aids" she/he will have no problems. Always keep in mind that a horse must be put on the the aids, by the rider correctly using them. Without which you cannot complete any correct work. The horse and rider can only achieve the goal through systematic and fluid work.