The crank pulley, or harmonic balancer, consists of two pieces of steel
- an inner disk and an outer ring - with a rubber band sandwitched
between them.

As the rubber ages, it is no longer capable of transmitting full torque
to the outer ring, so you get slippage between inner disk (driven by the
crankshaft) and outer ring (which drives the belts).

The symptoms usually start with a squeal on startup, and on revving etc.
Similar to belt squeal. This gradually gets worse, until the pulley
eventially comes apart.

A good way to keep an eye on it: using white marker, draw a couple of
lines on the outer surface of the pulley (the face you can see), running
from the center to the outer edge. These lines cross the rubber band. If
you ever get slippage, the lines will no longer line up. A quick glance
at the pulley at service time will tell you if there's been any slippage.