ast83, that is how I understand it as well. However, to expand on your good advice to always keep the descent speed in the FMC updated with ATC speed restrictions, it's worth pointing out that if the OP has reduced from say 250 to 220 early using Speed Intervent, and then updates the FMC that he will probably be in the region of 3-500ft high on the new profile. Of course early ATC speed restrictions could often mean its a busy day, perhaps delaying vectors coming up anyway, so probably no rush to correct this deviation from path as extra track miles may be forthcoming anyway.
However, if not and he does nothing to solve this (i.e. speed brake), upon selection of flaps, or through other activation the FMC 'on-approach logic' will cause the vertical mode to revert to VNAV Path, with a resulting pitch down and undesired increase in speed as the aircraft pitches to re-establish on the vertical profile.
As one gets to know the quirks of the aircraft you can anticipate this and prevent the finger mashing of buttons on the MCP...just use another vertical mode (presumably Level Change if you're already happy with a thrust idle VNAV SPD descent up until now) until you're happy that you're approaching your VNAV path, then re-engage VNAV. Or if you were below profile, do you really want to level off for a few miles while ATC were expecting you to continue your descent to your cleared altitude....use a low V/S.