The harmonic balancer is also called a "vibration damper". It sets up an opposing force to the twisting motion the connecting rods induce into the crankshaft as the cylinders fire by absorbing some of the energy in the rubber damper, and then snapping the outer ring back at a different frequency than the input from the rods. It's supposed to prevent vibrations and possible crankshaft breakage due to harmonics in the crankshaft. Some engines such as the Chevy small block 400 also use an out-of-balance flywheel and dampener to help balance the crankshaft since there's not enough room inside the crankcase for big enough counterweights on the crank.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!