Just to add a few details to the good stuff Bula has presented.
The managed speed is not a speed I can insert into the MCDU, and want the a/c to fly once I push the speed knob correct?
First of all, and this will answer one of your other questions, the FMGC will compute your descent ECON speed based on the following factors.
Cost Index, CRZ FL, ZFW, ZFWCG, block FUEL and performance criteria.
You can actually change this speed prior to the top of descent. For example, if ATC have issued a clearance that said 'When ready descend to FL200 and on conversion maintain 280kts, you could insert this speed into the DES PERF page. The FMGC would then compute a new profile based on this speed and this would be the managed speed target. On some of the older FMGC's once the aircraft commences the descent this speed cannot be changed. However, Airbus have just released a software update to allow the pilot to change this managed speed target in the DES phase.
Why does it choose to cruise at 250 knots when in managed speed? (for example).
Also, what is an "open descent, or closed descent"?
I think Bula has answered this one for you.
And "The other way you can do it, when you are in HDG, you could do a DIR TO the centre fix of the runway (NAV mode engages), then immediately pull HDG again, so then HDG is back on the FMA. This will in turn sequence the flight plan.".
I didn't quiet understand this method.
Ok this is a little tricky to explain without an FMGC trainer. Lets say for example the STAR points are as follows.
A to B to C to D then to the Final Approach point of the ILS typically coded CR27L (27L at Heathrow).
I assume you understand that your TO way point is located on the top right hand side of the ND in white and on the second line of the MCDU? Lets say you have commenced the STAR and are flying towards point B and have 30 miles to run. If ATC then give you a heading change of 30 degrees and start to vector you around the sky for the approach. If you do nothing with the FMGC, depending on how far you are from point B, it may or may not sequence the flight plan. Therefore you could be down wind on the approach with point B behind you, maybe 30 miles or so, and the FMGC bases all its predictions of you returning back to that point, as its still your TO waypoint. Looking at the ND, because you havent deleted in way points you will see the routing in relation to position of the aircraft. You know that you are not going to go back to any waypoints on the STAR, so it is not necessary to have them displayed on the screen, plus it will give you false track mileage and false time and fuel predictions.
So, in this situation, if you did a DIR TO the CF27L the aircraft goes into NAV mode and want to fly directly to that point. However, ATC want you on a Heading. So you pull heading again to maintain your clearance. What the DIR TO has done for you, it has made the CF27L you TO way point and will automatically delete all the other way points of the STAR, i.e. point B, C and D. As you are vectored onto the localizer the aircraft will fly over the CF27L point and automatically sequence the flight plan as the aircraft flies down the approach. This is important if a missed approach is flown so NAV mode will engage.
How Do I know it's activated? Is this the only time the LOC frequency / VOR or whatever approach it is will be tuned, and your final approach course set?
This is how you know your approach phase is activated?
First of all the ILS, VOR etc will be auto tuned before the approach phase is activated. The easiest way to know if the approach phase has been activated is to press the PERF button on the FMGC. If you have not activated the approach phase prompt in blue will still be there.
Good advice from Bula too, with regard to trying to get hold of an FMGC trainer for your computer. All this stuff will become clear, as it is difficult to explain it without having a visual demonstration.