The late David Charlesworth’s “Ruler Trick” is a wonderfully efficient addition to your sharpening methodology. For those unfamiliar with it, please see his short video – it is easier to appreciate than a verbal description. It basically is a simple way to hone the steel only just behind the cutting edge on the flat side of a plane blade without having to work the entire flat side. The resultant tiny bevel is only about 1/2°. (See the thin bright line in the photo above.) Thus, you create two meeting surfaces of finely honed steel to make a sharp edge.
It is often promoted primarily as way of efficiently commissioning a new blade. The “trick” is that you do not have to flatten the entire back or any sizable area of it, just that thin strip near the edge. For that alone, it is invaluable but it is even more useful in subsequent sharpenings to remove the wear bevel.
Here I must credit Brent Beach and his incredibly extensive and informative website. The “wear bevel,” simply put, is the rounded wearing of the top and bottom of the blade just behind the apex of the cutting edge. For both bevel-up and bevel-down planes, as the lower wear bevel develops, it requires you to push down harder on the plane to get it to cut. As the wear progresses, you sense the plane blade getting more dull. Notably, the lower wear bevel is greater for higher bevel-side honing angles in bevel-up planes, which are often used to reduce tear out.
When we sharpen the bevel side of the blade in bevel-down planes, we are rectifying that lower wear bevel. We remake two clean surfaces meet and reestablish the clearance angle just behind the edge apex.
A particular problem comes in bevel-up planes. That lower wear bevel is only addressed inasmuch as you effectively work on the flat side steel just behind the edge apex. If you do not fix the flat side, you have not effectively prepared the blade for work.
The practical reality is that it takes too much work to remove a layer of metal, albeit quite thin, over the whole back of the blade to remove the wear bevel on that side. Enter the Ruler Trick. It does it quick and easy. Brent Beach describes a steeper, more complicated “back bevel” but I think the Ruler Trick is a good compromise in practice, though I cannot quantify this.
So, here is my point: while the Ruler Trick is very helpful for sharpening bevel-down plane blades, it is, as a practical matter, essential for sharpening bevel-up plane blades. Note too, that many specialty planes are also bevel-up, such as shoulder planes and router planes. They need the Ruler Trick too.
Next: What about chisels?