Like the chainring, the teeth of the cassette are subject to tremendous torque from the pedalling force applied to the chain.
The chain stretches under the force of the pedal stroke and over time the teeth of the cassette are moulded by the shape of the elongated links. The result is a ‘fatter’, more ‘splayed’ profile on the teeth of a worn cassette.
Shifting will also wear the cassette. As the derailleur moves the chain, it pushes it up and over the teeth of one sprocket on its journey to the next. The result is a wearing down of the teeth. There is little that can be done to mitigate this type of wear, other than shifting less – an undesirable solution. Replacing the chain before it becomes excessively worn, however, will help to slow the wear rate of the cassette.
The effects of an excessively worn cassette do not have the same catastrophic potential of those from an excessively worn chainring, however. While you’re almost certain to experience a loss of power, and even a slight hesitancy before engagement in the power phase of the pedal stroke, even the most badly worn cassette is unlikely to unship a chain.