Ever heard of piano hands? It means a rider twists his wrists in such a way, the knuckles point down.
The problem with this being that you can’t move with the horse anymore. Your lower arm is no longer a straight line with the reins to the bit. You can’t feel the mouth and it is more likely you start pulling. There is this anecdote of the Spanish riding school in Vienna, where one of the horses didn’t do the half pass to the right so well. The oberbereiter told the rider to pay attention to the position of his outside hand. And he was right, as it was in piano-mode. When it was turned upright, the half pass was perfect.
Sometimes piano-hands happen because someone is tired. And a lot of the times when I see this the rider also hunches his shoulders forward. It helps to work on the core stability. Sit up, lift your ribcage, without holding your breath. See a picture in your mind of a dressage rider you admire and try to copy the way he or she sits. Roll your shoulders back and relax. Pretend you have two glasses of champagne in your hands. And it is delicious, so you don’t want to spill a drop.
If you are a recidivist, practice with the elastic band as described in chapter about hand position.