ARM, This is one of those situations where if you could see how someone is using the turntable it would make perfect sense, but trying to explain it is difficult.
When you look through the scope you see a very small field of view somewhere between 1-3 inches (2.5 - 7.5 cm.) depending on your scope and how much magnification you use. Imagine you want to engrave an object that was a bit larger like a knife. Let's say it's 5 inches long (13 cm.) so you mount it in a holder (thermolock, knife vise, etc.) and then clamp that into the vise. Assume the center of the vise is already directly in the center of your field of view. Now, if you mounted the knife in the vise so that it was perfectly centered from left to right it would hang out of the field of view in your scope by 1 inch (2.5 cm.) on each side. This wouldn't be so bad if you wanted to work on the direct center of the knife which at this point would be in the center of the vise in the center of the field of the microscope's view.
Now, what happens when you want to actually engrave the bolster by the blade? It's hanging out of the field of view. The next logical thing is to move the vise so that the part you want to engrave is under the field of view. The only problem with this is that when you go to spin the vise (to move the part into your graver as you spoke about earlier) the entire bolster will move out of view. The farther away from the center of the vise the larger the swing area will be. Imagine concentric circles like when you throw a pebble into a pond. Each circle away from the original point of entry is larger. In this case the direct center of the vise rotation is our epicenter.
Moving the vise and the scope to compensate for this becomes old very quickly. To compensate for this by the turntable you do this:
1) Put turntable under scope
2) Use centering peg (this comes with GRS turntable) or mark direct center of rotation on turntable (can be with tape, etc.)
3) Look through scope and put center of rotation of turntable directly in the middle of your scope's field of view.
4) Lock the rotation on the vise so it is stationary.
5) Put vise on turntable under the scope.
6) As best as you can line up the part you want to engrave on - in this case the bolster near the blade - directly over the mark on the turntable.
7) Look through the scope and fine tune the bolster into the field of view.
8) To check that it is right rotate the part while looking through scope. If it is correct you will see the part rotate, but it will stay in the view. If it is still drifting slightly take notice of which part of the object (in this case the bolster) seems to be staying still. That area is over the direct center the "epicenter" of the entire set up. Move the bolster closer to that point.
At this point you will only turn the turntable for the rotation. The vise top is locked. If you look at the setup the center of the vise will be about 2 inches (5 cm.) off of the center of the turntable center and will rotate with the turntable, but will travel around one of the concentric circles. The bolster should be in the center and should be relatively stable.
Now as you start to work, the tilt of the vise may have to be adjusted slightly - maybe you have a slightly curved bolster, or perhaps there is a clip or something to work around. With this set up you can "nudge" the bolster into the field of view with your left hand as you change tilt, etc. You will get very good at moving the entire vise into the field of view so you can keep working and not be chasing the part.
This movement on the fly is one of the reasons I'm considering a slightly lighter vise. The magna block can be heavy after a couple of hours of movement. On the other hand, it works with this set up and is very stable.
Now, when you have a very small part that is able to be centered in the vise, stop the rotation of the turntable (tape, pin, GRS sells a clamp, etc.), allow the vise top to rotate and just work off of the vise rotation. I leave the turntable on my drill press stand almost all the time. Sometimes I work with the vise locked and sometimes I work with the turntable locked. Either way it's ready to go.
My drill press table also rotates, but I keep it locked with only slight movement. I only use it to position the entire set up under the scope. When I'm not using the above set up, I use a the small vise (I call it the baby vise) in the bottom of my lowest pull out drawer on my bench. The drill press stand is off to the side and I can swing it in or out depending on what I need or how large the part is. If I'm doing large motorcycle exhaust parts I swing it out in front of the bench and work there so the rotating parts don't hit the bench.
Hope that helps.