Arm twist joints
To prevent collapsing on the forearm and for good wrist deformation you need two extra joints between the elbow and the wrist.
1.To make these joints duplicate (ctrl+d) the elbow joint twice and delete all child joints.2. Name these joints armtwist0_L and armtwist1_L and place them along the elbow bone.3. Parent them (p) to the elbow bone.4. Select wrist_L and armtwist1_L constrain>orient (only constrain the x axis) This will cause armtwist1 to rotate in X when the wrist is rotated in X.5. Next, select armtwist1_L , elbow_L , and armtwist0_L constrain>orient (only constrain the x axis)6.Now set the weights on the new orient constraint to 0.5.7. Select the skin and the two new bones and click Skin>edit smooth skin>add influence8. Paint weights for the two new bones on the arm.DONE!!! Now you have a forearm that won't collapse when you twist it, and a wrist that will deform correctly as well.