A BOP was being lowered by the hoist winch onto the studded cross on the X-Mass tree. The Roustabout (RA) along with two other crew members were trying to align the holes on the bottom flange of the BOP with the studs. As the holes and the studs were being aligned the BOP dropped by an inch. The RA’s hand slipped from the body of the BOP and his right hand ring finger got trapped between the stud and the bottom flange resulting in a fracture to the RH fourth finger.