"Idle", that means Open Descent on the A330 is a bit a pain. As long as you're descending with the speed in mach mode (selected or managed), the airplane chases its target speed with pitch inputs, which may result in overspeed. It's the same on the A32x, but it's worse on the A330 due to its inertia. Therefore many people use vertical speed for the initial descent.
And even when descending in managed mode, the engines are rearly at idle (IDLE displayed in the EWD). Thrust setting is always a bit above idle, even when flying above the profile.
During an "open descent", EPR drops below zero, and I might be wrong here, but as far as I understand the EPR definition, this means that the engine at Idle produces drag. So it might be more economical to start your descent a bit earlier with a small power setting than fly level and then have a steeper descent with more drag.
But I'm really not sure here on the last part and I'm open to any new input. I just perceived that with "managed descent" you get a shallower approach than with "open descent".