In climbing, what is a hand jam?

Hand jamming is a technique used in crack climbing used to support the climbers upper body. The climber places their hand in a crack, then flexes their thumb towards the palm to expand their hand, creating friction between the sides of the crack and the base of the thumb and back of the hand. There are many variations on the technique depending on the size of the climber, the size of the crack, and the circumstances of the route.