O Sensei doing suburi Suburi hyakkai refers 100 practice cuts with a sword.  Usually suburi hyakkai is done as a routine  either at end the day or start of the day.  Famous swordsmen like Nakayama Hakudo and Tiger Mori Torao supposedly did 1000 suburi every morning.  Sensei told us that Tiger Mori was so engrossed with doing his 1000 suburi every morning that his family knew never to bother him until he finished.  Despite all their accolades and fame, swordsman like Tiger Mori or Nakayama Hakudo did their suburi religiously because they were all chasing this idea of perfection.  They were never satisfied no matter how good they got or how many awards they received.  They knew that in order to perfect themselves they need a vehicle to not only gauge their state but to perfect it as well.  That method was the sword cut.

Supposedly one's sword cut is the representation of one's inner state.  If the cut is a reflection of one's inner state then what we are really talking about is one's inner mind or, more generically, the subconscious.  I am sure most do it either at the start or end of the day as a matter of convenience, but what they probably didn't know is that doing it at that time is the best time or way to access one's subconscious.  Sleeping is done at the subconscious level and doing suburi before bed allows your cut to sink into your subconscious.  Doing it right when we wake up allows us to access our subconsciousness.  Either way it gives us the feedback as to our inner state of mind.

A serious swordsman knows that the one true opponent lies not outside of himself, but lies within.  The outer work we do on ourselves pales in comparison to the work we do on our inner selves.  The sword cut is then a symbolic gesture where the opponent we are really cutting down is ourselves.  Therefore a serious swordsman has to do at least 100 suburi everyday.