A common diagnosis of problems with various kinds of reasoning is that they involve a confusion of ideas. What exactly is going on when we say that two ideas are confused? On Hume's account, the primary explanation of confusion of ideas is an easy transition between two separate ideas. This is not the most obvious way of understanding the problem. One could very well think, as the very term 'confusion' suggests, that confusion of ideas involves some sort of blurring of different ideas together. Another possibility that fits some of the ways we talk about this confusion is that it involves ideas that are by their nature interrelated in some way so that we have to distinguish them at a more abstract level. Another possibility is that ideas themselves can be unclear, so that they don't provide enough information to admit of a definite distinction. Explaining confusion of ideas in terms of facile transitions doesn't fit many of the ways in which we talk about confusion of ideas. And an obvious problem is that transitions between ideas are extremely common, so it's not exactly clear how much the transition theory explains to begin with.
However, Hume needs something like a transition account, because he can't accept any of the more plausible candidates. Two key principles of Hume's method are the copy principle and the separability principle. According to the copy principle, all of our ideas are copies of impressions from which they derive. According to the separability principle, everything distinct is separable. These two principles together sharply restrict what explanation we can have for confusion of ideas.
If all of our ideas are copies of impressions from which they derive, we can't attribute confusion to ideas themselves being unclear; each idea is just a copy of what it derives from, and there is no identifiable information loss. Hume thinks ideas are less vivid or forceful than impressions, but his entire method requires that we only genuinely have a kind of idea if we can identify the kind of impression it exactly copies. Impressions themselves, however, can't be vague or indistinct. There are no vague ideas in Humean empiricism. We can have words that are vague in the sense that it's unclear to what ideas they refer. Ideas themselves, however, don't admit of any internal vagueness.
If everything distinct is separable, we can't explain any confusion of ideas by the internal interrelation of ideas; as Hume repeatedly says, no ideas imply the existence of any others, all ideas are separable from each other. They only have relations in experience and custom.
Given both the copy principle and the separability principle, ideas can't 'blur' together, they can't fuse, they can't meld. Simple ideas are indivisibles of mind; they can't be related by overlay or by blending. So in confusion of ideas, we can't be blurring them together. And it follows from both principles together that we can't distinguish distinguishability of ideas from their actual distinction (this plays an important role in Hume's account of space).
Thus Hume has a transition theory of confusion of ideas because he doesn't have many other options, and the big thing he has to explain is how the kind of transition involved in confusing two ideas is different from other kinds of transition. Hume tells us (T 184.108.40.206 (SBN 60-61)),
I shall therefore observe, that as the mind is endow’d with a power of exciting any idea it pleases; whenever it dispatches the spirits into that region of the brain, in which the idea is plac’d; these spirits always excite the idea, when they run precisely into the proper traces, and rummage that cell, which belongs to the idea. But as their motion is seldom direct, and naturally turns a little to the one side or the other; for this reason the animal spirits, falling into the contiguous traces, present other related ideas in lieu of that, which the mind desir’d at first to survey. This change we are not always sensible of; but continuing still the same train of thought, make use of the related idea, which is presented to us, and employ it in our reasoning, as if it were the same with what we demanded.
This 'imaginary dissection of the brain' to some extent doesn't require the brain; that is, the point of it is that the when there is an easy transition between two ideas with respect to their objects, due to resemblance, causation, or contiguity, there can develop an easy transition between the two ideas with respect to our habit of having and using them. As he puts it more clearly in a later passage (T 220.127.116.11 (SBN 202-203)), using the most obviously confusion-relevant association, resemblance, as an example:
Nothing is more apt to make us mistake one idea for another, than any relation betwixt them, which associates them together in the imagination, and makes it pass with facility from one to the other. Of all relations, that of resemblance is in this respect the most efficacious; and that because it not only causes an association of ideas, but also of dispositions, and makes us conceive the one idea by an act or operation of the mind, similar to that by which we conceive the other. This circumstance I have observ’d to be of great moment; and we may establish it for a general rule, that whatever ideas place the mind in the same disposition or in similar ones, are very apt to be confounded. The mind readily passes from one to the other, and perceives not the change without a strict attention, of which, generally speaking, ’tis wholly incapable.
Neither dispositions nor acts of mind have a particularly well-defined place in Hume's account of mind, but this does give us an account of confusion of ideas that is consistent with the copy principle and the separability principle, and which at least gives us some kind of answer as to what distinguishes easy transitions that involve confusion of ideas from other easy transitions: the relevant easy transitions are those that allow us to think of two different ideas in the same disposition or attitude or manner of thinking.
It's a clunky account for what generally is taken to be a fairly simple phenomenon; it has the curious feature that you can never, strictly speaking, be confusing two ideas at a single time (there needs to be a transition), and another curious feature that you cannot confuse two ideas due simply to their newness or lack of familiarity, because the only confusion of ideas it allows is that which arises from already having developed a habit of moving from one idea to another. But it is perhaps the simplest possible account for someone who accepts both the copy principle and the separability principle.