To understand it a bit better you need to take a step back and consider "What is a managed descent?"
In laymans terms it is a descent profile, where by the FMGC has taken into account weight, wind, track miles, with speed and altitude constraints. It will actually plan a shallower profile to allow for deceleration, and allow for a steeper profile if needed.
When the approach phase is activated, it will sacrifice the vertical profile to attain its target speed, and will assume the pilot will continue to configure the aircraft to its landing configuration.
All this sounds good in theory, but practically I find it a little outside the realm of what would be considered real world profile and speed management. It does a good job, but is far from perfect.
Speed: Climb, cruise and descent speeds are specifically based on Cost Index and vary, depending on your airlines planned profiles. Essentially a trade off between time and fuel. The higher the CI, the greater the burn.
As for the 250 kt cruise. You are either below 10 000', so the airbus automatically assumes the standard speed restriction for most airspace applies unless you remove it manually.
Or your at 38000 ft or so, where you may be indicating 250 kts, but actually are targeting a specific Mach.
Open Descent, commands the ATHR to IDLE and ignores all FMGC vertical profile constraints. There is no closed Descent. You may call this a managed descent however.
May I suggest investing in a FMGC trainer and having a close looks the flight related phase of FCOM. This will help answer your question flight phase by phase
Last edited by Bula; 28th Dec 2011 at 16:17.