In my recap of DevCon 2009, I called Geoff Coffey the "God of Script Triggers." I hear that he rocked DevCon 2010 as well.
Geoff has recently posted to the Six Fried Rice blog about the OnObjectValidate script trigger (which is new in FileMaker 11). In his post, he uses a couple of very helpful diagrams that clearly illustrate how FileMaker processes field triggers.
Be sure to check it out, here: http://sixfriedrice.com/wp/the-onobjectvalidate-trigger/