THREADING A PIECE OF 3/4"" DONDUIT WITH A STUB 90ANGLE ON A RIGID THREADER. AS MOTOR WAS STARTED THE PIPE STARTED TURNING, THE 90 CAME AROUND AND CAUGHT INSIDE OF EMPLOYEE'S SAFETY HARNESS. HE LIFTED HIS FOOT FROM THE MOTOR SWITCH. IT FLIPPED EE OVER THE MACHINE AND ONTO THE FLOOR, WHERE HE INJURED HIS LEFT THUMB.