Resistances and evasions are bound to happen as you school your horse. There are specific corrections that will assist the rider in creating the intended movement but the main ingredient in any correction is that you must not allow the horse to change you not your body and not your mind. Since horses out muscle us at about 10 to 1 they can shift our bodies before we realize it ever happened. So if we cannot out muscle a horse we must out think them. This requires absolute confidence and precision in every movement you wish to create.
Riding is a dynamic activity and by this I mean action begets action you turn a shoulder and the horse turns her body and yes that is all it takes if all is right in the universe that is which of course happens far less often (by chance) than not. Both horse and rider will make mistakes but neither needs to be frustrated by such. To avoid over facing both horse and rider you must develop a system whereby you know your own position before you ask it of the horse and check back frequently to ensure that you have not allowed that position to be shifted.
In the beginning if you find yourself in an incorrect position you must release the horse, reposition yourself and start again. As you get better at maintaining your own position, despite the horses best efforts to shift you around, you can then correct both yourself and the horse while on the move. At times this will work and at other times it will not. If your attempts at correction in motion fail to correct the horse and the shifting becomes worse it will cause the rider to over-compensate creating tension and even more distortion throughout both bodies. Once this tension enters the picture it is almost impossible to make valuable corrections so as soon as you realize that your efforts are in vain I would rather have you release everything, take a deep breath, shake out the heebie-jeebies and reapply the correct aids. The horse will appreciate that you are not contorting yourself into a crazed pretzel and will respond, in kind, on some level.
Some typical evasions that occur during the learning phase of lateral positioning and their corrections are listed below. Please note that these are very specific corrections (not aids) and that the application of each should be released as soon as the horse even tries to comply with your request. Also note that they should be applied in addition to whatever position you were originally working on. This is a bit tricky at first but becomes easier with practice.
If your horse backs up you must maintain your position and seat do not lean forward. Leaning forward releases the horses back making it easier for him to slip it into reverse. If he continues to back the next step is to arch your lower back toward your hands. Backing as an evasion comes from confusion and frustration so please refrain from retracting your hands backwards at this moment or you will confuse him even further.
The haunches swing away to avoid moving the shoulders.
Swinging the haunches away when bending or working with shoulder-in can be corrected with a quick opening rein. The opening rein goes out to the side in the opposite direction of the haunches. Again no retraction backwards of the reins should occur and you must take care not to lean over toward your hand. As soon as the haunches start to swing back where they belong the rein returns to its neutral position and the dominant leg and body aids should take over. Even if the haunches immediately return to the improper position you must first release the open rein and then reapply it. Holding this rein position for too long will only create more distortion, a distraught horse and then, perhaps, an unseated rider.
Too much bend in the neck.
Bending the neck too much in a lateral position or a bend around a curve is generally a rider problem but even if the horse creates this distortion the correction is the same. The inside rein must be quietly pulsed and the neck needs to be straightened on the outside rein. As with all rein aids there should be no retraction backwards of the arm or hand. A firm squeeze of the fist is all that is needed your horse will feel this lightest of touches and respond if all else is in order.
As you get better at maintaining your own position your horse will evade less and less. As your horse gets better at maintaining her position it will make your job even easier. As you both step up the ladder of success first one partner then the other you will find all aspects of your riding getting easier. The horse becomes more sensitive to less aids and will begin some level of collection, engagement and lightening of the forehand on her own. These three qualities are coveted goals for all dressage riders and you have achieved the same simply by controlling your own body challenging for the rider you bet but the results are amazing.