Prohibiting the explicit disclosure of the hidden meaning is precisely to enable the manas to attain realization

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. This is because ready-made answers obtained from others, as well as answers produced by guessing and imagining, only correspond 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 consciousness, grasp every piece of evidence, and carry out verification and deliberation (manana). 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 cannot realize the truth, and without realization there is no liberating merit and benefit. 

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