From the video description:

In this video, we show a useful trick to helping you maintain a consistent angle while sharpening and diagnose problems in your sharpening. By using magic marker or sharpie, you can actually see where your sharpening is taking place. This way, you can gauge when your angle is too low (marker is still on the edge of the knife, but is being removed behind the edge), too high (marker is only being removed at the edge, but is smaller than the bevel), inconsistent, etc. It adds one more level of feedback you can get from your sharpening.

We recommend this for everyone from beginners to seasoned professionals. Its just helpful.

Remember to keep some nail polish remover around to clean off your knife when you are done.

If you have any questions, please feel free to contact us at Jon@JapaneseKnifeImports.com or look us up at www.JapaneseKnifeImports.com

Thanks.

-Japanese Knife Imports