Homemade Sausage Making

So I was watching Film No 1 on sharpening...as a woodworker this holds some interest to me. The film https://www.youtube.com/watch?v=-6AFZ_MvmY4

Since I am one of those people who until recently thought "kielbasa" was a specific sausage recipe, I have no hope of translating the video. Unlike the sausage making videos this one is tough to determine if they are saying "don't do this", "here are the pros and cons", "this is one way to do this", etc. Although I don't know all the answers as a tool user I'm sure there is some flexibility. However, there is one thing that happened in this video that I can't get my head around. If you go to approx minute 4:25 you will see the guy slide the newly sharpened knife along his thumbnail as if his goal was to slice it. I realize he puts no pressure on the blade and is only gently "dragging the knife". But what is going on? Is this a technique to "feel" if there is a bite to the edge? i.e., to detect any microserrations? Or is it a way to strop the final burr off the blade? I am used to using my nail perpendicular to the cutting edge and if the edge catches on the nail....it is sharp.

I would like to understand what they were teaching here.

It is one of old polish butchers school to check when blade of knife is smooth/sharp (without micro serrations). On that video time frame (3:19 - 4:25) - he check and told that blade has two places needed for burr polishing.

thank you Stefan.