Just like the leg position and your upper body, the way you hold your hands and arms is important for a reason.
The correct way is the only way in which your shoulders, elbows and wrists can move, so you can have an elastic contact with the mouth. If you sit upright, your upper arms should hang down naturally without any tension in your shoulders. Elbows against your sides without clinching and bend in such a way that your lower arm is in the same line as the rein. Wrists are slightly bend outward, hands upright in a fist, knuckles pointing towards each other. Thumps on the reins. Reins running between your little finger and your ring finger if you use a snaffle. With double bridle there are several ways to hold the reins, but I’ll get back to that when we talk about using snaffle or double bridle.
How far should your hands be apart? Hold your hands in the proper position. Stick out both little fingers. If they just touch, that is the right distance between your fists.
If you are riding a young horse that has no clue as to what is steering, it can be necessary to take your hand sideways if you want to turn. But this is just the explanation phase for a baby. No rider in Grand Prix makes a turn like that.
Some riders tend to hold their hands too wide apart. Usually they try to push the head of the horse down with their hands that way. But the rein is not the instrument to force a horse into the outline. If you ride your horse properly, the outline will come naturally. If you push your hands down, the snaffle is pressed onto the sensitive parts of the mouth. The result is the opposite of what you want: the horse will try to avoid this feeling and push his head up even more.
Hands too wide will easily result in pulling. The right hand position is invented for a reason. You need it to keep the elastic contact. And it helps you to stay in a balanced position. With wide hands your elbows go out. You can’t use them to move with the horse in that way. And you will hunch and lean forward.
When I started to learn to ride with a double bridle, my hands had to be really quiet. So I used a little trick to teach myself to keep them in the right position. It feels very awkward, but the sooner you learn to do it the right way, the sooner you can do without. Mind you, it took me almost half a year and I still use it every now and then, just to make sure it’s still okay. So here it is: take an elastic band. It shouldn’t be too long. I used one of those to make a ponytail. Put it around your little finger, hold the reins in the proper way and hook your other little finger in the band. Like I said, it feels uncomfortable. It’s all about self-discipline. But that goes for a lot of things if you want to be a good dressage rider.
Another good test is to see if you can get your horse to chew the bit and move his head and neck downwards, without pushing your hands down or wide. Be patient, try not to help him in any way. The only thing you are allowed to do is ride a little forward. You might think this won’t work without doing anything with your hands, but it is the forward flow that should make him want to reach for the contact, not your hands pulling him in the front.