秘传的，神秘的，难懂的。

Difficult to understand. Also: of special or unusual interest.

常规不是那种秘传的东西但确实是冥想的秘诀。

Routine isn't that esoteric but it is the secret to meditation.

武当山内家功，为祖师爷张三丰所创。以桶子功、千斤坠、五毒手为独门功夫，自宋代秘传至今。

Wudang internal martial art, created by founder Zhang Sanfeng, famous for bucket practice, kink-weight, poisoned hand, taught secretly from Song Dynasty till now.