Thomas, for me the burr formation is the knives steel edge indicating I have done my job correctly and apexed the edge. That is I have brought the bevel angle, as I set it, up and then off the knife edge. When I have accomplished that on both sides of the knife properly, completely and thoroughly the results is the burr where the two bevel planes from either side of the knife intersect. The burr is the only only true physical indicator we get that the bevels have been apexed properly.
The vertical up down strokes employed are done on small portions of the knife edge, in an alternating, side to side method, to keep the amount of effort balanced, on both sides, in order to maintain even height bevels. I work one side section for a littler bit, then switch to the same section on the other side for the same amount of time. This attempt to balance my work is verified visually with a lighted magnifying visual aid. I choose to employ a USB microscope and a laptop computer.
I do the sectionalized, scrubbing walking the sections forward and backward along the knife edge till the scrubbing is complete and thorough. While walking the scrubbing it’s blending the scrubbed portions together to create a continuous scratch pattern across the entire knife length on both sides of the knife.
The appearance of a parallel scratch pattern laying side by side and uniform in appearance and angular direction, along the entire bevels on both sides of the knife, from shoulder, up across the bevel and off the apex, indicates my efforts are thorough and complete and it’s time to move on to another stroke or the next grit, (depending on where I am in my sharpening progression). The sequential scratches, grit after grit, are a visual indicator that our physical efforts are being performed correctly and consistently. The bur, in addition to these scratches, indicates the scratches have apex the knife edge.
The burr formation comes and goes. It is flipping back and forth side to opposite side as I do the scrubbing, alternating the knife sides. As the burr is formed, it works it way with my up and down scrubbing along the edge from side to side indicating my effort is thorough.
Is this necessary? All I can say is it’s the results of my properly executed method, this one particular stroke, of my multi-stroke sharpening technique.
IMO, I feel like too much negativity suggests to sharpeners that burr formation is a bad thing. That it is something that we should strive for just once and only once. Then all subsequent sharpening till the very finish should be done to the point just short of drawing the burr. In reality the lesson I believe has been twisted and misunderstood. The burr doesn’t need to be avoided. It just needs to removed, if it’s formed, so the remnants don’t remain on the sharpened knife edge.
Many sharpeners form the burr and continue to form the burr again and again with subsequent grit work, never removing it and allowing it to pile up and build. This results in what appears as a well sharpened knife and a well polished bevel that isn’t very sharp because the burr is intact on the very tip of the knife edge.
Sharpening is a sequential repetition of consistent activity grit after grit, over and over, to take the initial edge profile, the angle we choose for the bevel, and smooth it so the intersection of these two opposite side flat bevel planes intersect precisely and keenly to a thin line, the pointed sharpened knife edge. It can be done any way you like. Any way you can figure out how to accomplish that. The more effective and efficient your method is, the sharper, quicker and better polished appearing results your technique will yield.
In summary: I employ a scrubbing stroke, first, with each and every grit, to establish or re-establish the bevel angle profile. The burr forms as a result that this is done thoroughly. I follow the scrubbing stroke with an alternating side edge leading stroke to remove the formed burr and to smoothly shape the apex. Then I repeat this again and again, grit after grit, with attention to the results in a consistent manner. After the last grit in my sharpening progression, I strop the sharpened knife edge.
Everybody can achieve similarly sharp results as those of us that have figured it out and have been able to put our personal techniques together. You can do it any number of ways, that you choose. I really prefer you do it differently then I do. That way, as you struggle to learn how to do it you won’t blame your learning difficulties on me. The bottom line is it takes time with consistent repetitious efforts with attention to detail.
