The reading is one of the four main skills in the language. It is also the one, that can help the language learning the most.
Why do I think that? Because the reading is the quickes way to get input in the language, and the only way to produce a language, is if that was already heard or read as an input.
Intensive reading is the type, where the greatest advantage is the leaps of knowledge, that happens with it.
When reading intensively, I check every word, I am not completely sure of the meaning. I sometimes check the grammar rule, that was used in the sentence. Even more frequently, I try to deconstruct the rule from the sentence, and then note it down. If I am across the same structure in the text again, I can check my hypotheses.
I do vocabulary intensive reading every day. That means, where I only check the words, I am not sure about.
Only sometimes I do the grammar intensive reading. Checking the grammar of the sentence or seeing the rules, or looking for the structure, that I recently learned about.
Sometimes I do, what I like to call the style intensive reading, but is not really about the style. I just work through the text, noticing how the words are used together. Or if there is some phrase or sentence, that I think was really masterful written. Or some clever vocabulary.
All of the above are some form of intensive reading. As long as the text is processed with goals in connecting with the language itself, it can be called intensive reading.
I hear about the people, that go through the text, checking for everything at the same time. I find it too slow, so I tend to concentrate on the one aspect. Usually the aspect, that I think is stopping me from advancing.
For vocabulary intensive reading, I am using the program called Learning with Texts (link on the end). It is a program, that remembers the words, that are saved and I can use it to see how much of the language I understood. If the word is white, I had marked it as well known. If the word is dark read, than I marked it not understood. The blue means, I had not encountered it in the intensive reading yet.
What I did found out is, that intensive reading helps with the fluency only at the lower levels, and then the advantage of the fluency diminish. But in case of vocabulary study and grammar, it can be helpful even in my mother tongue. The vocabulary did increase in all 5 of my language with it. Also, sometimes I I used this one native texts in all the languages that I know on the regular basis. But I am doing it more, if it is the language I am concentrating on right now - and that right now is German.
Here you can get to the Learning with Texts webpage.