Traditional metal sharpening steels have been around for over 100 years. They work in 2 ways to re-align the edge of a knife. 1. Sharpening steels are made out of a very hard metal. Usually 4-5 degrees harder than the knives. The "harder" metal of the sharpening steel help push the cutting edge of the knife back to strait.
2. Magnetism. Sharpening steels are magnetized wixh helps pull the grain of the edge back to strait when using the steel.
The texture of sharpening steels vary from coarse/regular to ultra smooth or no texture at all. For standard kitchen use, a regular cut sharpening steel will leave a strong cutting edge with some bite. For butchering meats, or cutting raw sushi a fine sharpening steel will leave a smoother cutting edge, that will provide less drag.
Oval or flat is better than round...always. Oval sharpening steels provided 4-5 times more surface area for sharpening. Oval steels work much faster and leave a much sharper edge than a round steel.
Japanese Knives and Metal Sharpening Steels? There may not be a perfect answert to this.