One popular phrase that accompanies marketing literature on decorative imitation Japanese weapons is that the blade features a "blood groove" or "blood channel". While certain cultural blades may design fullers/grooves into the blade to supposedly allow blood to flow out and to reduce suction upon the blade by the body wound, in the case of Japanese (and Chinese) swords (as well as European) this is not necessarily the case.

The actual purpose of the groove or hi (pronounced "hi") in the Japanese blade is to lighten the mass without too much compromise to the structural rigidity of the sword or knife. In the case of swords, since the Japanese sword is primarly a slicing weapon and not so much a thrusting weapon, there is no real need for a "blood groove". Thrusts can still take place without "suction". (This however does not discount the possibility of body weight exerting itself over the blade, e.g. the opponent hunching over the wound - but this is an entirely different matter.)

Though Japanese blades tend to surround a small number of styles, within each style are a plethora of subtle differences. For example, some designs call for a wider blade, but this would normally mean a heavier sword. Carving grooves in the shinogi-ji (or blade flat, above the ridge line - in the case of the shinogi-zukuri style) allows for a lighter sword. This may create the illusion of "strengthening" a blade when in actual fact there is less mass to lend to lateral bending. In other words the blade will appear stiffer but it is actually structurally weaker. These compromises are factored in by an experienced sword maker.

Some hi may stretch throughout the blade, and some may stop half-way or stop some distance short of the tip. There may come a point where the presence of a groove will compromise the strength of a blade too much if it were to continue (especially if the blade becomes particularly narrow).