The rig crew was preparing for a rig move and the mast had been laid down. The Injured Person (IP) was attempting to rig up the wire slings at monkey board. While the Injured Person (IP) was arranging one sling at monkey board his foot slipped down about 3 FT (1 meter) causing his body to twist and his left hand caught on the upper finger of the monkey board. This resulted in an interior dislocation of his left shoulder joint. The IP received medical treatment (his shoulder was put back in place), and returned to work. In the worst case scenario the IP could have experienced more serious body injuries or fatality.