The master finally imparted the innermost secrets of swordsmanship to his disciple.
The essence of the tea ceremony cannot be understood merely by learning the procedures.
極意 is the secret, ultimate technique or wisdom of a specific art or skill, while 真髄 more broadly means the essential nature of any subject.
Composed of 極 (きょく/ごく, 'extreme, ultimate') and 意 (い, 'meaning, mind'). The combination refers to the ultimate meaning or deepest intention behind an art or discipline.