Zen Enlightenment - the result of sudden insight, in depth study or intense practice?

To understand the essence of Zen Enlightenment, a student does not need to go beyond the Platform Sutra. What I am talking about is intellectual understanding and not the actual attainment of such Enlightenment, which is another thing. Any further reading with the objective of a better understanding will likely to yield diminishing returns, except writings (like this post) that aim at promoting a better reading of the Platform Sutra.

On the subject of actual attainment of Zen Enlightenment, any reading (including reading the Platform Sutra and this post) cannot deliver the results. Enlightenment is a human experience rather than intellectual understanding, as I shall explain below.

In the Platform Sutra, there is one important example of Zen Enlightenment, as delivered by the Sixth Patriarch Huineng himself.  When the former general Hui Ming caught up with Master Hu Neng, the former was shocked out of his worldly-self (ego) into a statement of personality suspension (between ego and Unconscious) by the following definitive enlightenment statement:

惠能曰："不思善，不思惡，正與麼時，那個是明上座本來面目?"

Huineng said: When you are not thinking of doing good, and not thinking of doing bad, at this particular state,  what is your original self, my honorable Ming?

The above is not too difficult to comprehend.  Our original self is one that is not subjected to any karma influence which shapes our thinking.

A few questions: How is a person being enlightened when in such a state? Can a personal be in that state 24/7? Can a person enter such a state at will? How does such a state having anything to do with our physical existence where karma rules?

When in such a state, Honorable Ming was in the reality of Emptiness (空), a situation in which we know the fullness of the situation yet we are void of any cause/effect or karma influence. Decision making (in our normal logical sense) is impossible when in such a state, which, when our mind is active, raises the possibility of change. Psychologically speaking, we are out of our ego, yet we are not at the mercy of our Unconscious (in dreams, our Unconscious raises its (sometimes ugly) head). For honorable Ming, he got it and he was immediately out of that state, and told Master Huineng that he got it! Perhaps we can all have moments of entering that state in our lives, but our minds are just not prepared for THAT. Anyway the answer to the second question is obvious: one cannot (and need not) be in that state 24/7.

His Holiness the Dalai Lama once told of a Zen koan appeared in Tibetan Buddhist literature. Once a learned Zen-master from China visited Tibet. The Tibetan Buddhist monks welcomed him and his followers to live in a famous temple. They took good care of him and learned from him. One day (being naughty), the Tibetan monks wanted to test the learning of one of the Zen master's students. They asked him, referring to the teaching of Huineng "Isn't it that by being 'not thinking of doing good, and not thinking of doing bad', you'll be in the state of enlightenment."  He answered yes. "Isn't it in your Zen teaching a practitioner should be in the state of enlightenment in walking, living, sitting, and sleeping (行住坐臥）?" He answered yes. "Doesn't it follow that one should not be thinking of, and therefore not doing, good (deeds) all the time?"

I should not try to explain the koan here. Anyway, any explanation of a koan, like this one, that is worth-its-salt cannot be fully explained by any singular explanation without contradicting oneself. The gist of the matter is firstly one has to recognize (and be prepared for) such Enlightenment opportunity. After one is being enlightened, he doesn't need to seek other opportunities. He got it then he got it; he didn't get it then he didn't get it. What he needs to do is to learn (a practice) how to create that state at will.

In the Platform Sutra, the following statement clearly talked about the secrecy of deep meditation:

惠能云："與汝說者，即非密也。汝若返照，密在汝邊。"

Huineng said, "What can be said cannot be a hidden secret (teaching). If you look inside yourself, the hidden secret (teaching) is within".

Now the way of practice is deep meditation and the details of which were not mentioned in the Platform Sutra. Deep meditation is a private affair and has different routes that the depend on the particular characteristics of an individual. The omission of such details in the Sutra is totally needed and understandable. So much so Vajrayana whose foundation rested upon deep meditation is also called Secret Mantra and in Chinese it is named as such: 密宗.

Where to practice? Master Huineng suggested Ming to practice among the mountains.

In conclusion: How can a student of Zen Buddhism get his enlightenment through more and more readings?

The preserved body of Master Huineng