The forearm has two bones: the thick radial bone and the thinner ulna bone. They are not straight; they are curved so they may wrap around each other to allow the hand to rotate from a palm-up to palm-down position. When punching a solid object, these bones are located between the resistance of the target at the fist and the driving force of the body at the elbow. As a result, when under these stresses, they bend away from each other.
Because the ulna is thinner than the radius, it bends more easily and more extremely, producing uneven support at the wrist. However, the body compensates for this unevenness. Between the radius and ulna is a thin, but tough, connective tissue called the interosseous membrane. As the bones bow apart under stress, the membrane is drawn tight, transferring the stress between the two bones to keep them from bowing further and maintaining the structural integrity of the forearm. The force of the punch is distributed over the length of both bones so there is virtually no loss of power or injury. However, the interosseous membrane does not transfer force laterally; it transfers the force diagonally between the bones.
The radius is quite thick near the wrist but thin near the elbow, while the ulna is thin at the wrist and thick near the elbow. The angle of the interosseous membrane ensures that the weaker portions of each bone attach to the thicker portions of the other bone. And finally, the angle of the membrane is designed to work with the natural stress alignments along the arm when a punch is delivered, which is typically force generated along the line of the radius. The pressure on the radius will cause it to move back slightly, but the angle of the membrane works to keep the bone in place and transfers the forces without any play or slack. This means one should be careful not to punch strongly with the middle, ring, and little fingers because they direct the stress of the punch along the ulna.
During a full-twist punch, the ulna and radius wrap around each other, which means they collapse onto each other and the connecting membrane is folded between them. With the membrane slack, it cannot support the structure of the forearm bones, so they bend apart and shift upon impact, producing unequal forces in the wrist that results in a loss of punching power and a risk of injury at the weakest areas of the bones. Conversely, during a three-quarter-twist punch, the position of the bones maintains optimal tension in the membrane throughout the movement.