by 
The Curious Bag
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It's hard to say when the first harmonic balancer was used, but these simple steel-and-rubber pucks have prevented crankshafts from cracking and engines from shaking themselves to bits for decades.
Piston Force
When a piston pushes down on the crankshaft, it imparts a certain amount of force that is not converted to rotation. This force manifests itself as crankshaft twist.
Engine Harmonics
At certain RPMs, the piston force can "synch" itself to the vibrations produced by those in other cylinders, causing cracks to appear in the crankshaft once it reaches the metal's resonant frequency. The principle is similar to shattering a crystal glass with a high C note when no other note will.
Harmonic Damping
Since the rubber inside of a balancer has a much higher resonant frequency than metal, it can absorb and dissipate any resonance in the crankshaft to which it is attached.
Mass Resonance
As pistons can fire out of synch with each other and shake the engine, balancers use a heavy outer ring to resist rapid acceleration and deceleration (shaking) of the rotating assembly.
Removal
Some people insist that removing a harmonic balancer is good for a gain in horsepower. You can ask them if it worked when you see them walking home.
Source:
Resource:
Video: Removing an Harmonic Balancer
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