I know I've seen a better image, but this gives you the idea:
http://home.xnet.com/~site/spi/wafer123.jpg
The wafers are the gold things. The key fits through the slot in the wafer, lifting it to a particular height so the outer part of the wafer retracts into the plug (against the pressure from the little spring), allowing the plug to turn freely.
They are easily opened with rounded-tip picks, so a snake rake will do the job.