you give the edge and bevel a painting with the magic marker, it helps to see if you're making contact with the very edge when honing, say someone has set a bevel and honed with tape, or an old wedge with multiple bevels. basically you'll see a very thin line on the edge left if the edge isn't making contact. I also use it as an aid to figuring what pressure to use when bevel setting and honing, helps me keep an even bevel i.e. do i need to use a varied pressure due to uneven spine wear do i need to do my patented tape compensation trick etc