THE PISTON IS HELD AT BOTTOM DEAD CENTER BY FIRMLY HOLDING THE PROPELLER TO PREVENT THE ENGINE FROM TURNING WHEN AIR PRESSURE IS APPLIED THROUGH THE DIFFERENTIAL COMPRESSION DEVICE TO THE COMBUSTION CHAMBER. USE GLOVES OR RAGS TO PROTECT THE HANDS WHILE HOLDING THE PROPELLER BLADE. ALSO, BEFORE ATTACHING THE COMPRESSION TESTER, CHECK THE AIR SUPPLY REGULATOR TO MAKE SURE THE AIR PRESSURE TO THE CYLINDER IS NOT EXCESSIVE. AIR PRESSURE INTHE CYLINDER CAN CAUSE THE PROPELLER TO TURN. KEEP CLEAR OF THE PATH OF THE BLADES.