DISCLAIMER: The video above was published on YouTube by Trace Study. The video is part of an amazing series of videos featuring prof. Kenji Komatsu from Toyama University teaching about the setting and maintenance of a Japanese plane (kanna). I do not own, nor I claim, any rights on these videos - all copyrights are owned by the respective owners.
Let me show you something that we need to worry about when sharpening a kanna blade. When we sharpen continuously in the same position certain things can happen.
Before we found that the blade was not flat here. If we sharpened continuously without hammering from the back, after a while the blade would eventually be flat along the edge. The problem, though, is that the width of the shiny parts on the two sides of the blade (ra ashi) would also become wider.
If possible we need to try to keep the ura ashi as thin as this one. The only way to achieve this is by hammering to avoid excessive sharpening.
Another thing that can happen if you move the blade always in the same position while sharpening, is that a step will form all along the width of the blade. You can see that the shining part here ends abruptly. Like this… here is a small step. From the back you can see that the ura is shaped like a gourd.
This one is too wide. We perform uradashi to keep the ura thin.
Another thing to keep in mind is that we need to sharpen just the edge, like this. Some people have the tendency to sharpen with almost the entire blade resting on the plate, because it becomes more stable. Here is the result: too deep. This is not good.
Here is another thing to be aware of. If you keep the position of the blade too much inside the sharpening plate/stone, here you can see a step. This is because it's too far away from the edge
Finally, if you don't support the head of the blade while sharpening, the back is heavy so it tends to fall and press along the edge of the plate. The result is that you get a pattern shaped like a gourd.
So these are some "not good" examples.