Step 5: Thin the Edge
You're just thinning the area around the edge a bit to save you some labor later on.
In each of the following sharpening steps, you'll raise the angle just a bit.
That way you're always shaving the stone with fresh metal.
Commentators to this howto are rightly pointing out the merits of a 20 degree knife edge, (knife held at a 10 degree angle to the sharpening stone) or a 17.5 degree wedge. I think my homemade plane blade sharpening fixture is set at 27.5 or something nerdy like that.
Use your own numbers, not mine, and by all means get carried away with your own refinements.
The numbers I picked aren't too important, just that you raise them with each step.