I agree with this post except for the turning your wrist. You aren't supposed to "break" your wrist, it should stay straight. Your wrist alone is weak, but your arms are stronger with your thumbs up and wrists straight, in case your horse bucks, bolts, refuses to turn, etc.When we hold our reins correctly with the fingers closed and gripping the reins and our thumbs on top, a straight wrist and the index finger more forward than the pinky we are able to better hold a contact and be soft at the same time.
This is because the two bones in the forearm are then parallel and do not cross, when the wrist twists and the forearm bones cross our forearm becomes rigid.
Also, when our hands are in this position it allows us to flex the horse by simply turning the wrist, instead of makeing a backwards motion with the entire arm.