Gennou – that’s what the Japanese call these hammers. Selective hardening of the striking surfaces means that part of the recoil energy is absorbed by the somewhat softer core of the steel. You feel you’re working more effectively, also because less of the recoil energy makes its way back to the wrist joint.
The octagonal hammer head have one flat and one slightly convex striking face. Use one or the other face, depending on the job at hand. The flat face, for instance, is used for tapping chisels or for driving in nails close to the inside edge of a workpiece. The cheeks of the hammer head glide over the surface of the workpiece, so you can be sure that the striking face is exactly at right angles to it. This provides you with the best chance of knocking in small nails dead straight even in an inconvenient location. For setting nails you use the convex face of the hammer to minimize the risk of damaging the surrounding wooden surface.
In Japan, metal hammers are the tools of choice even for chiselling work.