Book of 5 Rings
The heart of Musashi's ideas is the mind. The Mind should be pure, uncontaminated by self-serving distortions and preconceived ideas. The mind should be flexible, able to change shape according to the situation without resorting to rote learning. This is why he described his ideas in the water book (). Water is pure and flexible.
About footwork, Musashi says we should always move right-left right-left. Never move just one foot. The implication is that we should have a correct, balanced stance and always assume it. To move only one foot would affect our balance and ability to move suddenly. It effectively decreases the value of our starting point for any future action.
When holding the sword, we need to maintain our method regardless of wether it is a practice, an execution or a battle. The implications of this are obvious.
Further, he says we should maintain flexibility in our grip. Flexibility is life, inflexibility is death. You have to think he is paraphrasing Lao Tzu's Tao Te Ching, written in China 2,000 years earlier. To quote:
Living plants are flexible,
In death, they become dry and brittle.
Therefore, stubborn people are disciples of death,
but flexible people are disciples of life.
He would certainly have been aware of the Tao, and could have read it easily, since the old Chinese writing style still heavily influenced Japanese writing in the 1600's.
The Water book also has some classic aggressive Musashi quotes. My personal belief is that Musashi exaggerates proactive, aggressive ideas, because the defensive ones will come naturally and even dominate you when your life is on the line. By starting out to dominate and destroy the opponent, we will find a balance between defence and offence naturally, rather than starting out with defensive thinking that can turn to fear, driving us to run away.
Form your own opinion. As Musashi says: You should study this yourself thoroughly!
"It is critical that you think everything is an opportunity to kill."
Importantly, after explaining stances for sword fighting, he explains that on a larger scale, the placement of people is equivalent to a stance.