Triggers are one of those categories that I haven't stumbled into much until recently. They allow you to take a Quicksilver noun and verb combination and bind it to a key chord. For example, I issue the noun ~/work/PRDPRG, verb Open with..., and adverb Mate a lot (this opens my Productive Programmer work directory in TextMate). The only problem is that it's a little cumbersome to do that over and over. So, I set up a trigger for that noun, verb, adverb combination and bound it to the CTRL-ALT-1 key. Now, when I hit that keyboard combination, it issues that sequence of Quicksilver commands automatically.
Quicksilver: sleight of hand