Boeing VNAV Path
Normally opening the speed window changes VNAV Path to VNAV SPD(at least on my type) and the path is no longer followed. But when on approach using VNAV, the speed window can be opened to adjust speed while remaining on VNAV Path if certain conditions are met such as flaps having been extended.
I was also told that you have to be within a certain distance of the "approach", perhaps 25 miles. Is this true?