bear,
Just got one of those X-dies myself in .223. Haven't used it yet, but what it does, is it stops the growth of the fired case. You must trim your brass prior to using the X-die, but the instructions say that it is the only time you will have to do this. Call RCBS, and they will send you the instructions. They are excellent about that.
Don't know there number off the top of my head, but it is an 800 number, check a gun magazine.
Steve H in AZ