What a Friend We Have in Jizōs 13

The reason why the shape of the sword had to change was simple- cavalry had become the predominant fighting unit. The older straight chokutō were unsuitable for fighting from horseback. The curved sword was a far more efficient weapon when wielded by a riding warrior, because the curve of the blade added considerably to the downward force of a cutting action. An organized motion made by arms and wrist, this descending strike had its own specific technique, and name.
Ten-uchi. As the sword was swung downwards, the elbow joint drastically extended at the last instant, popping the sword into place, causing the swordsman's grip to twist slightly, like wringing a towel. The nihontō’s blade impacted its target with sharp force, breaking whatever existed in the way of initial resistance. From there, fluidly continuing along the motion wrought by ten-uchi, the arms followed through with the stroke, dragging the sword through its target. Because the nihontō sliced rather than chopped, the ‘dragging’ is what did the maximum damage, and the reason why it was incorporated into the cutting technique. Due to a combination of the motion and its curved shape at full speed, the swing appeared to be full stroke, the segments hardly visible, if at all. Ten-uchi broke the initial resistance of a target human torso supplied by shoulder muscles and the clavicle, and the follow-through continued the slicing motion through whatever else it encountered, until the blade inherently exited the body.
The great thirteenth century Mongol invasions, only narrowly averted by the Kamikaze Divine Winds, drove the next change in blade shape. Samurai, forced to abandon mounted archery for hand-to-hand combat, found their swords too delicate and prone to damage, when used against the thick leather armor of the invaders. Japanese swordsmiths, appearing all over the countryside, began to make blades with thicker backs, thinner and simpler temper lines, and bigger points, in adaptive response to the Mongol threat.