OPEN DES...you are 100% correct...
by pulling V/S to shallow the descent in order not lower than the computd profile then pulling OPEN DES ....is correct if you have normal descent path with out any constraints.
But...if you have a speed constraint over a waypoint (let's say to cross a waypoint at a speed of 250kts)...i think it is better to drop below the profile rather than selecting V/S to shallow the descent...because you need some more distance for the speed reduction...as the FMGC was calculating a speed reduction from 280 to 250 not from 320 to 250.
and as for the ALT constraint....no problem at all...it will be respected either with selected speed or managed as long as the DES mode is engaged.
thanks ...
please feel free to comment