The aji machine
The aji concept is one way of dealing with the huge range of possibilities in go. Aji discussion is a way of organising some half-chances, dreams, over-the-horizon effects that can be seen in a position. One needs this sort of peripheral vision to become strong, rather than just trying to set up a big contest in one part of the board and win it.
The 'machine' view of aji is that it can be 'processed' - converted from one type to another - without destruction. One wishes to conserve one's good aji and remove one's bad aji (see aji keshi). This requires skill, in bringing enough aji together in one situation in order to make some concrete use of it (to live by combining enough fractions of eyes, to carry out a tesuji in a place where the opponent has several weaknesses).
Ko fights tend to consume aji - if they go on a long time, much of the aji in the rest of the board may be played out in clear-cut ways. Ko threats that actually create more aji than they destroy are likely to be specially interesting. One way they occur is when the opponent has a choice of answers, and chooses the greedy go approach of making them look loss-making threats in points terms.