This isn't complicated. The pinch bolt is the lower one in the photo that the cable runs through. It's a screw with a hole through it just below the head through which the cable runs. When the nut is tightened the bolt draws up against the brake "pinching" the cable. Freeing it is simply a matter of loosening the nut (front in the photo) and pushing the screw back a bit so it's no longer "pinching" the wire. However the wire will still be distorted and may not want to slip easily, which is why I suggested using the brake lever as if applying the brake to yank it back.