Recording my personal observations about the forearm. Note that these are just my personal notes based on things I've observed by looking at multiple pictures from my anatomy book, and I'm not sure they are 100% correct, though they appear to hold true for all the cases I've seen. I'm using my anatomy book as reference for the anatomy. I couldn't find these exact poses of the arm and hand, but found some pictures that were close enough.

The ulna and radius seem to remain fairly stationary as the forearm rotates around the area where they connect to the humerus. The axis of rotation appears to pass through the individual centers of these two bones around these areas, and thus the bones simply rotate in place around the elbow without shifting position. At the wrist, however, the two pivot axises of these bones appear to converge around the center of the wrist, and so the position of the bones change primarily around the wrist where the pivot is no longer centered inside the bones, but outside the bones at the center of the wrist.