EMPLOYEE AND 2 CO-WORKERS WERE MOVING A 12' SEC-TION OF 4"" PIPE BY HAND TO INSTALL A FLANGE. PIPE 12' WITH A 9D AFTER 8'. 2 EMPLOYEES WERE STANDING UP PIPE TO SLIDE IT OVER, WHILE INJURED HELD BACK VALVE; HELPER PULLED ON PIPE BEFORE THE OTHERS WERE READY, PINCHING INJURED'S THUMB BETWEEN PIPE AND VALVE AND CUTTING TIP.