Originally Posted by Nige
You pose a very good question.
The reason that the demarcation that you question will not occur is that one only dodges using this mask for "a portion of the print exposure time". Obviously if the high density sharp mask were printed in register with the camera negative throughout the entire print exposure the effect would be to totally block exposure to the affected area. If one uses a registration system that is accurate then the affected area is affected in a very precise manner.
Conversely if one does the cruder method of making a mask using opaque then the best method would be to make a mask the size of the final print and the fashion a "jiggle frame". The "jiggle frame" is a frame that allows the mask to be positioned over the print in a manner that the frame is supported at each of the corners by a weak tension compression spring. This allows the frame and mask to be "lightly jiggled" throughout the period of dodging or burning operation. If I were inclined to try this method, I would make the frame in such a manner that the frame would be built to my largest print size and in which the support for the mask could be adjusted downward in size.
I hope that I have answered your questions. Please feel free to address the matter again if you have further concerns or if I have failed to adequately communicate to you.