Obviously opinions vary, but I will give my 2¢ worth:
Standard theory and many people's experience says that, assuming you already have stream entry, that as the Review phase progresses, and cycling through from 4th-11th ñanas to Fruition gets easier, many will begin to notice subtle layers of mind and experience that seem to not be well perceived, seem to be outside of their current level of mastery, seem to contain unskillful elements that the practitioner had had some respite from in the afterglow of their current realization, seem to be newly noticed, and then there will be a period where there will seem to be a choice: do you focus on the current stages, which by this point generally seem easy, or delve into those things that seem to be beckoning for investigation and clear perception?
There is actually no choice, as there is no self, and reality is causal and happens on its own, but the basic sense of the choice can present anyway. As one becomes less fasciated by the review stages and attention turns more to those patterns of experience (which may initially be subtle, but for most will relatively rapidly grow in predominance in the field of experience), the initial insights that one had for the first path begin to be noticed, by way of the same type of investigation that landed the first path, and so the early stages of Mind and Body, Cause and Effect, etc. arise.
Now, it may be that during this phase Fruitions may still occur, and regression to previous stages may set in. However, as the dharma leads one on, through careful investigation the A&P will again arise, and after the Dark Night hits, one will generally feel more cut off from territory that is familiar and well-mastered, Fruitions may be vague or seemingly inaccessible, and, once the same insights that got on Equanimity are re-obtained for the new territory, Equanimity and potentially the next path arises.
However, it is not always so simple, and some will get caught at the standard places, both pleasant and unpleasant, by failing to have the investigative skills necessary for progress or by fascination by the good and bad aspects, and/or the lack of maturity to handle the new territory, and so fall back or stagnate, and people can get caught in between the paths at times, unable to get back to Review, unable to move forward, and eventually one moves one way or the other, and if that direction is back, then re-mastering Review can strangely help progress, and if one moves forward, then avoiding the standard traps again is of value, but finally, by good, inclusive, clear, standard practices, one attains to the new path.
It is worth knowing that beyond second path, there are some paradigmatic aspects that, if not understood, can block progress, and while standard practices can eventually overcome these, some heads up as to what to look for specifically can be of value, such as the width of 3rd, etc.
From first to second is generally pretty straightforward in comparison to what comes later for most.
I submit this in the narrow context of the paths early paths and working within that conceptual framework,
Daniel
