The Prohibition of Explicit Disclosure of the Secret Meaning Is to Enable the Manas to Realize Enlightenment.

All dharmas in Buddhism that require realization belong to the secret dharma. What is meant by "secret dharma" is that these dharmas must be personally investigated and personally verified by oneself; one should not hope to obtain ready-made answers from others, nor should one use the conscious mind to guess and imagine. Because ready-made answers obtained from others, as well as answers produced through guessing and imagining, all correspond only to the conscious mind—the conscious mind may easily understand and grasp them, but the manas cannot understand or grasp them. For the manas to understand and grasp, it must go through the process of investigation; the manas must personally participate in the conscious mind's deliberation and verification, understand every detail transmitted by the conscious mind, grasp every piece of evidence, and carry out verification and deliberation (manasikāra). Only when the evidence is sufficient can the manas deliberate clearly and know the ultimate truth. If the manas does not know, it does not realize; if the manas does not realize, there is doubt; with doubt, one does not realize the truth, and without realizing the truth, there is no liberating benefit of merit.

Therefore, in Buddhism, not only is it strictly forbidden to openly state the hidden meaning of the Mahāyāna dharma, even in the Small Vehicle dharma it is forbidden to openly explain contemplative practice (vipaśyanā-caryā) in overly specific terms, so as to avoid the situation where the conscious mind knows but the manas does not. If it is openly stated, the conscious mind will quickly understand it and will no longer be able to investigate it; without investigation, the manas cannot understand it and cannot realize it. If the manas has doubt and cannot resolve it, then there is no liberating benefit of merit.