I do not think that you can turn this dialog off. I have looked for a way but I have not found it yet. As for what it is doing, AutoCAD is turning your vector drawing into a raster image by redefining the dwg to a format the printer can understand. i.e. It will take a line drawn at 10.0,12.5 to 10.0,25.5 and change it to a line on a 11"x17" paper, it will be black, its width is .05mm, it is drawn 1" over, 1.12" up to 1" over, 2.37" up (this is the general idea). I think AutoCAD basically does not want you trying to enter another command while it is doing these calculations.