A log was being loaded onto a truck with a grapple. Because of the log's size, the grapple operator could not see the back of the truck and needed assistance placing the log in the truck. An employee climbed onto the back of the truck in order to assist. The log started rotating as it was being placed onto the truck. When the employee jumped out of the way, their hand was pinched between the log and a part on the back of the truck. The employee's left pinky finger was surgically amputated.