Eeks expressions are mysterious to me I just know a little about how they work. I know in the Inside Lightwave 6 book it covers expressions in a fairly lenghty chapter just never got around to useing them.
Here is a couple links to some expressions tutorials.
http://www.3dfightclub.com/newtek/tu...tsAndBolts.mov
http://www.newtek.com/products/light...ar-expression/
http://www.richardbrak.net/tutorials/copter/