The harmonic balancer basically dampens out some of the vibration associated with reciprocating engines. Without one, your engine will literally shake itself loose. It also helps to keep the drivetrain balanced and revolving smoothly. There's a couple of ways to keep the engine from turning over when you go to torque it down and loosen it. My buddy and I chock the back tires, mash the brakes, and put the car in fifth gear...utilizing a great big breaker bar and about three feet in extensions. If you have the pan pulled you can wedge a two-by-four in between the block and your crank (we did that twice when the engine was out). You can jack the car up and have a buddy hold the harmonic balancer with a strap wrench. Or you can buy a special tool (the part number is in your helms... Moroso also sells one for about 65 bucks) that holds the balancer, while you crank on the bolt with your torque wrench/breaker bar.