MattKing is correct. Here is a picture of my head
You can't see it in the picture, but you slide the bottom slider to the correct paper, then you dial the yellow knob left or right to select the number. The selection is a smooth transition, not a click to each varying filter. The numbers go from 0, .5, 1, 2, 3, 4, 5. In theory, I could choose something between the numbers to have a slightly different mix of blue/green than directly on 5. There is also a lever on the side to enable unfiltered white light to come through.
Thanks for all the wonderful detailed responses. I guess I was lacking a bit of understanding on exactly what the contrast numbers meant in the first place (regardless of what color the light is). I've never spent much time in the darkroom and don't even have experience with graded papers. I work in the graphic arts industry, so I have a good grasp on additive vs subtractive color theory. I suppose the Ilford manual spends more time explaining that since its much harder to wrap your head around. My head is actually much more intuitive. low number = low contrast, high numer = high contrast : )