It's very simple in practice. Understand that the dial - the green part in Matt's photo - rotates with respect to the white page. You look up a development index in a table, such as 40 for Tri-X in D-76 1:1, or 33 for Panatomic-X in Polydol. Then set the "normal" arrow to the development index on the left, and read off time vs temperature on the right. If you don't know the development index but have a manufacturer's recommendation or a time for 68F/20C from the Massive Dev Chart or whatever, you can set that time+temp on the right and infer the development index, and read off different time/temp combinations.
There are some extra tables with numbers to add to the index to adjust time (basically, adjust contrast) for condenser / diffusion enlargers, paper grades, and so on, but I mostly use it for the time/temp adjustment.