Instructing Advanced Drivers
If you read instructing novice drivers and instructing intermediate drivers, much of what applies to advanced drivers should fall into the logical progression from novice to intermediate to advanced.
The first step with advanced drivers is identifying their deficiencies. Unlike intermediate drivers who often will have many deficiencies - or opportunities for improvement - to work on, with the instructor selecting the most pressing deficiencies or the most beneficial improvement opportunities and concentrating on those, with advanced drivers the number of either will often be very small. It is possible to go through all suggestions inside a single session.
I generally expect advanced drivers to work on multiple things at the time. Whereas novice drivers often work on 1 or 2, and it is often preferred to work on conceptually the same or similar things at any given time, I expect advanced drivers to make changes to their driving as quickly as I can communicate said changes. In practice, an advanced driver might work on something every other turn, for a total of 5+ distinct things per lap.
Another advanced driver ability I expect is immediate retention. Once the driver takes a corner a certain way I expect them to continue driving it the same way for the remainder of the session without further instruction. This permits moving through the list of deficiencies/improvement opportunities very rapidly.
Advanced drivers typically can turn fairly general suggestions into actions. For example, an advanced driver hearing "brake later" should normally be able to identify a new reasonable braking point on their own.
Because instructors typically have a lot of experience in a small number of cars and comparatively much less experience in other cars, when instructing advanced drivers instructors would often give suggestions for the driver to try rather than commands meant to be unconditionally executed. When giving these suggestions the instructor may in fact not know if they would prove to be beneficial. The expectation, again, is that the driver will be able to try them without losing control of the car and evaluate whether the change is worthwhile.
I often find it impossible to give real time commands ("turn now") to advanced drivers, as the tolerance for error becomes less than the driver and the car's combined reaction times. In these cases I would give relative suggestions ("turn later"), counting on the driver to figure out a meaningful but not excessive change to their inputs.
Most advanced drivers are capable of discussing a corner as they continue driving, both through straights and through subsequent corners. I am often able to go into why I am suggesting a certain approach while driving on track. At the same time it is sometimes more efficient to bring the driver in the hot pits to have a long discussion - advanced drivers often recognize the net benefit of stopping and talking over continuing to drive suboptimally.
I try to always have my AIM Solo when coaching advanced drivers. As many times the changes I suggest imply making different tradeoffs, data is necessary for validating that the new tradeoffs are better than the old ones. A typical example of such a tradeoff would be time in the corner vs corner exit speed, and therefore time in the following straight.
Advanced drivers tend to understand compromises and tradeoffs and have an ever increasing range of things they are able and comfortable to experiment with.
Advanced drivers often benefit greatly from riding with instructors, as they are usually able to figure out themselves what it is that makes the instructors faster.
On Track Communication
Advanced drivers generally have a pretty good idea of what they are doing. Unlike novice drivers who get commands and intermediate drivers who get commands with explanations, advanced drivers get suggestions. While the speech may be similar ("turn later"), the difference is in semantics. When I say "turn later" to an advanced driver, what I typically mean is "turn later and let's see what happens, maybe where you turn now is better after all".
As pace is much higher in advanced run groups, the suggestions are often less precise and rely on the driver to complete them into action items. For example, I would try to give a specific turn in point to a novice driver - e.g., "turn at the end of the curb" - but an advanced driver might just get "try to turn later".