A katana has no moving parts. All its lethal energy comes from the positioning of the hands. The hands allow the blade to move in different ways, saving time, producing unexpected angles of attack and allowing it to cut or thrust in different directions with minimal effort. The Four Assassins workout goes through the four grips. Practice changing hands first, to get the idea and then perform the moves slowly, so that you get a sense of the different muscles that come into play and allow the sword to move faster, in tighter angles. Once you have practiced it the change from one grip to the next will star becoming easier and your katana swordplay will have gone up a level.
Extra Credit: 30 seconds rest between sets.