Introverted Thinking (Ti) is the attitude that beneath the complexity of what is manifest (apparent, observed, experienced) there is an underlying unity: a source or essence that emerges and takes form in different ways depending on circumstances. What is manifest is seen as a manifestation of something. From a Ti standpoint, the way to respond to things is in a way that is faithful to that underlying cause or source and helps it emerge fully and complete, without interference from any notion of self. The way to understand that underlying essence is to learn to simultaneously see many relationships within what is manifest, to see every element in relation to every other element, the relationships being the "signature" of the underlying unity. This can only be experienced directly, not second-hand.
Introverted Thinking leads you to relate whatever you are doing to some larger principles that you have identified. Hence, Ti is like having some kind of book in your head, which describes the inner workings of things. When interacting with reality, you are constantly writing and re-writing your book. To deal with anything, you have to be able to understand in terms of the observations in your book. Whenever you are dealing with any new system, you start writing a new chapter on it in order to attain complete understanding of it.
This approach may seem very cumbersome from an extraverted standpoint. Youd don't really need to understand how a bicycle works in order to ride one. You don't have to actually understand a subject in school if you simply cram and memorize. You don't have to understand computers to check your mail. Yet Ti leads you to desire complete understanding of whatever you are doing, instead of looking up the correct procedure, or asking your friends for help, or kicking it when it's not working. With Ti, you don't simply try to understand a system well enough to manipulate it. You try to become such an expert on how it works that you could write a book about it if you had to, even if your expertise is unusable or useless to everybody (sometimes even to yourself).
Hence, Ti is a kind of high-bandwidth understanding, because it leads you to try to understand the entire causal, aestethic, or logical mechanism of any system of interest. This kind of understanding takes much more time and effort to develop, but it is more flexible once attained, because it allows you to deal with aspects of reality that cannot be described through social norms or sets of discrete procedures.