Musashi explains many different techniques in his book, but there are a few themes that run through the book.
Timing is choosing the moment to make your move and it is understanding the opponents' rhythm. To move in slowly and then speed up will throw off the opponent's timing. Ensuring you are in striking distance when you want to strike is timing. Arriving at the strategically correct time is timing. Knowing when to act on a decision you made is timing. In sword fighting the sword amplifies the subtle movement of breathing. By watching the tip of the sword (kensen), you can attack while the opponent is breathing in. When breathing in, it is almost impossible to attack. Also, for example, if you understand the opponents' rhythm and speed you only need to be slightly faster than them to win. This conserves your energy/resources.
Musashi was skilled in the arts and crafts. His observations of these activities were applied in his techniques. for example in the ?? Book, he explains that carpenters find a use for every kind of wood, even the apparently useless and similarly we can find a use for every person under our control. Importantly he never lost sight of his identity. He was Samurai. He made art, but was not an artist.
Musashi frequently says his ideas are scalable to large groups:- explaining in a one to one metaphor and applying it to controlling tens of thousands.
The power of observation was a primary skill to Musashi. This meant noticing everything, but also observing in an objective manor. This helps explain his interest in Zen and Tibetan Buddhism which concentrate on being able to see the world as it is. Musashi knew our desires and fears colour our perceptions of a situation
Possibly the purest expression of this idea comes late in the book where Musashi explains that some schools of martial arts have special methods of jumping twisting and contorting to gain an edge in the field. Instead, he says, keep your mind and body straight and force your opponent to contort their mind and body. This is Musashi's way.
This should be first and last really. Musashi says again and again:
You must practice this well.
This can only be understood by practice.
Practice this well.
You can't learn this stuff reading a book. Only an opponent can test your understanding. Reality will strip away your erroneous ideas.
A deeper analysis of the book itself:
Book of Five Rings - One ring at a time