Hi Toc,
The crayford is a pleasure to use due to it's smooth motion, no backlash, and the fact that most have adjustable clutches and a lock.
The barrel is driven by bearings and pressure rather than a rack and pinion gear method.
Here is an interesting link for you to read. It explains how a Crayford works, it's history and how to make one.
http://www.aqua.co.za/assa_jhb/new/Crayford.htm