Ni vs Si memory recall: an explanation of Ni "context shifting"
To explain "context shifting," I shall introduce a database analogy to demonstrate how Si vs Ni "remembers" things. It's not a perfect analogy, since human minds are far more sophisticated, but it should hold for this limited instance.
Database tables have what are called "indexes": an index allows a very fast lookup of any particular data in the table that references that particular index. Often, if a query is running slow, the indexes aren't set up in a way that helps the query, and can be reworked a bit.
The analogy: consider a typical "memory" as being something that stores a "noun" and a "verb" and other miscellaneous properties such as adjectives, and so on. I shall focus on the "noun" and "verb" fields.
Si has an index on "noun" but not on "verb"; Ni has an index on "verb" but not on "noun."
How does this play out? I'll give a simple example using "cellphone" and "access the internet" as examples to describe the behavior; this is not to indicate that either cognitive ability is incapable of understanding either side, but is kept simple so that one can observe the "kind" of data and "kind" of lookup being done.
Si can do a "name" lookup on "cellphone", and immediately know what a cellphone is and does. (Ni does, too, for something so simple as a cellphone, this is just to be illustrative, not definitive.) Si would find under "cellphone": make phone calls, call 911, call my best friend, send text messages, and maybe connect to the internet and send emails and so on. What is stored is all the experiences one has had with a cellphone, and those cumulative experiences are an understanding of what the cellphone is for and how it can be used. This is a very fast lookup for Si.
Ni can do a "verb" lookup on "connect to the internet", and immediately generate a list of possible ways to connect to the internet, such as computer, router, wireless connection, cell phone, VPN, ISP, DSL, cable, and so on. This is a fast lookup for Ni. Si can do it, too, but it's not the same thing, it's not "fast," especially when one gets to obscure topics/activities.
When faced with a particular issue, Si immediately knows what "it is" (noun lookup), and works from there. "What it is" is constant, and does not change, and one's experiences with "it" determines the options available.
Ni, when faced with a particular issue, will have a particular "verb" or "action" or "goal to achieve" in mind, and internally look up on that basis. The goal, the task is the constant, not the thing, the "it." One's experiences with accomplishing similar tasks is what determines the options available.
So let's say one needs to send an email, but one's computer is failing to connect to the internet. Si will tend to consider the problem to be one of fixing the computer so it connects to the internet: some aspect of "the computer" is incorrect, and correcting it will make it work right. Ni, on the other hand, will often consider the problem to be one of "I need to connect to the internet to send an email." Obviously, if there is some requirement that specifies fixing "this computer" to connect to the internet, then one is constrained to do so within the computer, but in this case Ni regards the goal as sending the email. Ni does a lookup on "I need to send an email," and the computer is offline, Ni will pull out the cell phone, in a pinch, without even having to think about it, because this is where Ni's mind lives: it remembers multiple ways of accomplishing the same thing. (Si won't dismiss the option if mentioned, but may become preoccupied with fixing the computer instead of resolving the send-an-email task that prompted the observation that the computer didn't connect.)
As powerful as Ni might seem from this example, there are drawbacks. For example, I remember that there is a tool to sync databases, and I know exactly how to access it and run it, but for the life of me, I cannot remember its name. I know the name is correct when I hear it, but I don't think of it in terms of its name, it's noun, but in terms of what it does, in terms of what it is used for, its purpose. (It's meaning, perhaps?) In general, most of life's problems don't require the flexibility of the Ni approach: the Si approach is more direct, and will usually find a solution faster for any issues with which one has had experience. In my "send an email" example in the prior paragraph, it might actually be faster and more efficient to fix the computer's connectivity problem (it's already booted up and running, after all), which might just be a matter of a few practiced mouse clicks, and one can send an email no problem, rather than slowly type out a long email on the limited cellphone keypad. An alternative method is not necessarily better than the straightforward approach.
My main point isn't which one is better, but rather how their typical approaches differ. Each has practiced remembering things in a different way, and the speed at which the memories appear is dependent on how the entities of the memories are "indexed." One's understanding of the entities isn't lesser or greater: rather, the "shortcuts" one uses to get the right information for the task at hand differ.
Now, about context shifting (yes, that was a long analogy): Ne/Si will tend to hold names/definitions/what-it-is to be constant, but allow for rearranging such entities into a new configuration; Ni/Se will tend to hold verbs/actions/purposes/goals/problem-to-be-solved/what-I-need-to-do to be constant, and switch out "what-it-is", or especially in the Ni case of trying to figure out "what it really is", keeps on "redefining" "what-it-is" without explicitly saying so, based on a constant understanding of how it needs to "work."
So Ni/Se will regard Ne/Si as "changing the topic," because Ne will bring in new items not previously under discussion. Ne/Si will regard Ni as "context shifting," when Ni switches around entities or definitions to suit our understanding of the behavior involved, especially in a highly abstract, theoretical topic for which Ni's understanding is undergoing rapid development. It seems like "changing the topic" or "context shifting" because the underlying thought processes are "fast," often making decisions or references "without conscious thought", so explicitly explaining why one considers one's statements as relevant to the original topic can be difficult.
I'm still working on refining this analogy/explanation, so others' thoughts and contributions are quite welcome.