wld be easier on the eyes if you watch it in full screen mode. BAsically the man is saying that you convex the secondary bevel. At no time is the edge touching the abrasive material .You cant add metal to a flat bevel or concave.
I still think that a rounded primary edge is possible with a few light strokes as after an acute angle sharpening say at 15 degrees, being very very thin, a few light gentle strokes will change the geometry
btw there are more you tube videos if you do a search on convex grinding.
figure out what you want to achieve and "micro grind" correspondingly with teh right materials and backing to attain it hopefully.
I suppose the objective in this video is a better cutting performance. IF sharpening is all about being the smoothest cut might just as well sharpen at 10 degrees adn spend abit more time honing or removing nicks and chips..
All things being equal (and they rarely are) a fully convexed blade will generally be stronger, and it works especially well cutting some materials. I have a couple of kitchen knives that are fully convexed, and they feel very different in use from a typical beveled grind -- very buttery, if that makes sense. There's a smooth, polished feel that I like for trimming meat (esp. silverskin) -- somehow I get cleaner cuts and more control. A fully convexed edge is not nearly as good, though, for something soft like slicing tomatoes, which benefit from a edge with more bite.
And, of course, any hand-sharpened knife is going to be convexed to some degree, especially if (like me) you finish on a strop.
Even with my fully convexed blades, though, if I need to thin behind the edge, it's a lot faster and easier to do with a water stone. I then follow up on sandpaper/strops to refine the convex arc.