Niten Ichi-ryลซ. Musashi broke with convention by using both the katana (long sword) and wakizashi (short sword) simultaneously.
This was unheard of. Samurai usually held the katana with two hands for power. Musashi realized that holding a heavy sword with two hands restricted movement. By training one-handed, he doubled his offensive and defensive capabilities.
"You must not have a favorite weapon or technique. To become overly reliant on one thing is a weakness." Musashi taught that there is no fixed stance in combat; you must simply cut the enemy in the way that the moment requires.