All engines experience crankshaft torsional vibration.  After internal combustion, the travel force of the piston and connecting rod is so intense on the power stroke that it not only rotates the crankshaft, but actually twists its metal.  In many instances the crankshaft twist is an astounding one degree or greater! 

During a revolution, once the rod and crankshaft assembly passes bottom dead center comes up on the exhaust stroke, the force is relieved and the twisted metal rebounds.  Like a spring, this extremely violent twisting and rebounding action sends vibration in both directions down the crankshaft causing metal fatigue.

RPM changes the frequency and magnitude of the vibration.  In a typical inline 6 diesel engine, 3 cylinders combust in one rotation of the crankshaft.  That means at 2,500rpm torsional vibration occurs 7,500 times per minute!

Without a crankshaft damper, or with an improper design or worn out one, these vibrations would keep oscillating back and forth through the crank.  When they quickly begin to collide with other vibration frequencies of the same magnitude it amplifies and resonates a destructive force.  When these crashes repeatedly happen it results in worn timing gears and chain, irregular valve train lash, destroyed main bearings, fractured accessory pulley mounts and even crankshaft failure.
A worn damper is the leading cause of broken crankshafts.