One thing I am going to say and I think it goes with the whip mechanics thing. (For anyone interested in understanding what whip mechanics is about, I would search NextLevel's posts and maybe videos from Brett Clarke).
My experience with controlling the grip with the index finger and thumb and keeping the grip relaxed is, you don't have to consciously think about different grips for different shots. The position you need to put your forearm and wrist in for different kinds of shots causes the angle of the grip to automatically shift to work for that shot. So.....you don't change your grip but your grip does change and adjust for each different shot. You just don't have to think about it or attempt to do it consciously because the way in which the index finger and thumb hold sort of has to switch when one's forearm and wrist are in different positions.
As a result of a relaxed grip which allows whip mechanics, racket speed can be much faster without more effort. The natural shifts in grip help this process.
I guess, a last detail to add: the trouble with being "taught" "what grip to use" or what grip to use for specific shots is, grip can be very personal and part of this has to do with the fact that, we all have arms that are different, the bones of the humerus, radius and ulna are different for each person on the planet, the left and right arm bones of each person on the planet will have, at least, slight structural differences. Simplifying the information significantly, the angle of your forearms when they are relaxed and hanging by your sides will not be the same from left to right. Your wrists on the two arms will flex and extend at slightly different angles. These issues increase through our lives as our bones deal with stress differently on each side.
So, since, anatomically, we are all unique, it would make sense that, a starting place for grip in TT will likely not be where you end up. and if you are playing for years, you may develop how you hold the racket over time without ever even thinking about the subject.
Simplifying once again, if you looked at how Ma Long, ZJK and Waldner all hold their rackets, you would see differences for all three. But, likely, you would see grip differences for different shots as well if you looked at the grip of any one of those players through a vast number of shots.
Often with the relaxed grip discussion, information about firming or tightening grip on impact is also discussed. I have a feeling that is also best left to happen naturally without conscious effort.
When I watch slow motion of baseball players making impact while hitting, you can see, that, without thinking about it, baseball players will be very relaxed and then certain muscles will engage on impact. I think in any kind of racket sport, this will happen as your skill develops. Although, in TT, you do get interesting effects on spin, when you force the grip to stay relaxed on impact instead of letting those engagements to occur.
I have heard Der talking about that concept many times. So, I guess it depends on what effect you are going for. But for speed and spin from a loop (power) your body should know what to do when you have performed the stroke with a decent level of skill enough times.
And this is why we practice.