Hammilton,
You have the basic idea right. The details, however, are rather picky. Length of staff, length of cords, location of pivot, amount of weight, all affect the release. If you look around, there are designs published by people that have experience, or you can start small and learn how to do it the way they got their experience: trial and error.
Here's a sample design:
http://www.ripcord.ws/plans/plans.htmlHere's a whole site dedicated to trebs:
http://www.trebuchet.com/And an article from Scientific American magazine on how it works:
http://www.deremilitari.org/resources/pdfs/trebuchet.pdfOr if you are into online gaming and simulations, go design, build, and test one here:
http://www.globalspec.com/trebuchet/OK, enough of this, I gotta go ... design a treb. See ya!