Urasuki -Back plow-

Single edge knives have a steel back, and this steel is rarely a perfect plane. The ridge and edge, which is the steel around the contour of the knife, are left intact, while the central portion is thinly scooped out, creating a concave shape on the back of the knife. Because of this curvature, when the scooped-out portion is placed on a flat stone, it doesn't make contact, making it unsharpenable. This hollowed out and curved portion is called the "ura-suki". Conversely, when the back of a knife is placed on a flat surface such as a whetstone, only the outer edge, not the ura-suki, makes contact with the stone. Here we call the steel on the back along the edge "ura-ha" and the steel on the back along the ridge "ura-se".

However, it's not always guaranteed that both the ura-ha and the ura-se will make even contact with the whetstone, as the blade can warp or twist. That's why it's important to check for wobble before you start sharpening. If there is any part of the ura-ha that doesn't touch the stone, it means that only half the sharpness can be achieved. Minor problems can be corrected by applying pressure to the back of the ura-ha, but if the knife still doesn't make contact with the stone no matter how much you sharpen it, it's best to consult a professional knife sharpener.

Related websites
Enhancing Japanese Knife Functionality with Urasuki Dimples | KIREAJI