Make sure your arm is completely straight. This is how must people fuck up their wrists. Imagine there being a steel splint on either side of your forearm to the end of your fist, keeping your arm straight. If you bend it, while you put 50+ lbs of force on it, you can seriously damage the bones and cartilage giving you arthritis when you get older, as the cartilage depleats. If you have a bag, start slow, like maybe 5 minutes a day, as your hand and wrist bones get used to the force exerted on them they will become stronger. Just dont over-do it when you're starting out.