For a simple prototype, this is pretty fun! I like the smooth feel of moving the katana, and the controls in general, though I wish there was some effect attached to using the quick-turnaround, whether visual or sound. Sound cues for the incoming balls to make it easier to know when a ball is coming would be nice too, as well as perhaps making the balls slower and increasing their speed as the game progresses, since some pretty supernatural reflexes are needed to get some of the balls. The game clicked much better once I started using the quick-turnaround key more often, though as mentioned it is a little disorientating to use.
Overall, pretty promising for a prototype. Could be a really fun game with some more polishing of the difficulty and some better indications given to the player to make it less disorientating.