It's better to think of DES NOW as activate descent phase now.
The VNAV decent phase however only works with an active end of descent point and you can only get that by selecting a runway from the data base or selecting a runway and rwy ext, thereby creating the end of descent point.
Once it knows where to finish it knows where to start and VNAV will work.
Edit auto correct
Last edited by CCA; 8th Jun 2016 at 08:48.