An employee was observing a machine that was not advancing a plastic sheet correctly and jamming as a result. The employee was checking the tension of the drive belt by depressing the belt with his fingers when the machine unexpectedly advanced the sheet, causing the drive belt to move. When the drive belt moved, the employee's left index finger was caught between the belt and pulley. The employee's left index finger broke at the knuckle near the hand (proximal phalanx) and was lacerated in the same location, damaging the top tendon.