Kesa gake is a maneuver common in traditional kenjutsu (lit. sword skill). It is characterized by a hard slash in one direction, followed by a twisting of the blade and a return slash in the opposite direction. The technique is a natural follow-up as it allows one to minimize the amount of “dead time” where a swordsman would be vulnerable after having executed a technique.
The kesa gake poses an interesting problem to swordsmen in that it places a great deal of pressure on the tang, the “bottom” of the katana blade which extended into the sword handle. The tang usually has two holes (sorry, don’t know the technical term) through which a pin would be inserted, thus firmly connecting the blade and the handle. Sword handles were subject to not a little torque, oftentimes resulting in a snapped blade.
Now metallurgy in Japan had progressed far enough so that swordsmiths could either mold a relatively soft steel that could withstand the rigor of battlefield use (a tremendous amount of force is placed on the arch of sword when cutting through objects), but the drawback was that the edge of such a sword would warp easily, resulting in a dulled blade. Conversely, a harder blade could be fashioned, at the expense of brittleness. Such a sword would shiver into pieces if it was hacked against an object sufficiently hard. For this reason, a compromise was reached: the inner arch of a sword would be cooled more slowly (usually by covering it with a mixture of water and clay) resulting in a softer steel. The edge would be hard and would keep its edge, even though it still cannot chop through wood as it is commonly portrayed in popular culture. (Many traditional schools that taught bo and jo were developed especially to fight swords. Any bo or jo that came into direct contact with a sword would not only come out of the encounter intact, but would crack the sword.) In any case, to make a long story short, a sword with a defect could often break at the critical moment in a fight.
Perhaps it is for this reason that many swordsmen preferred the bokken (wooden sword) over the metal sword. The bokken was a one pieced weapon, and it did not break or snap under battlefield stress, and it was rarely as costly.
Note: Now that I look at it, I seem to have strayed considerably from the original topic. The information, however, (I hope) is still valid. Please message me if you spot any technical goofs. I am no swordsman (or swordsperson for you feminist types), and some of this information was obtained from online sources (including E2) that can be challenged for dubious credibility. My primary source, however, was Dave Lowry’s Bokken: The Art of the Japanese Sword.