Alan, When you're cutting in the lower portion of the vessel, the
3/16th is cutter positioned as if it's an extension of the hook in the bar. About the same as tools such as Jordan's that don't swivel. For cutting in the upper part of the vessel, the cutter is swiveled counter-clockwise a bit so that it does a better job of contacting the wood and therefore cuts better. How much it's swiveled should be a matter of experimenting. I suggest that you practice by turning a vessel with a wide opening so that you see the cutting action. Of course when the opening is small you can't see the action at all.
You'll find the the 3/16th cutter is more agressive than the teardrop scraper is, so that it removes wood faster. I use it for my initial cutting.
When cutting inside the vessel you should be cutting on-center or a little bit above center. Never below center. You'll get a smoother cut if you tip the cutter down a bit. The pistol grip allows good control of this.
Good Luck, Wally