Someone else here will have to find you the references as my A330 manuals are 3000 miles away and last time I flew the aircraft was over a year ago.
How managed descent really works:
In the A/C STATUS page (I think that is the name?) there is a "perf factor and idle factor" field. The idle factor is usually /+1.3 or some such number.
In THR DES from altitude, the plane plans a descent at descent speed, at flight idle PLUS a little bit based on the "idle factor". That is why in managed THR DES you may see .900 EPR / 40% N1 while in OP DES it is .850 / 35%. So based on the descent winds you have entered, it draws a "line" from the T/D point to 10,000 feet or your first altitude constraint, whichever comes first. The "line" is really several FPA segments attempting to give you descent, at the programmed speed, at idle thrust + idle factor. This of course factors in repressurization if required, winds etc.
The airplane gives itself 20 knots on either side of the chosen speed, and will allow the speed to vary with the wind to maintain the planned descent "line". You will see it gently pitch up and down, but the descent angle and power setting remain constant and comfortable.
In the event you get high on profile and the speed reaches the fast end of the "window", power goes to THR IDLE as the plane "dives off" the altitude. In the event you are low on profile, the plane will "stretch the glide" until reaching the low end of the "window". At that point thrust will go to SPEED or MACH, and increase to maintain the bottom of the window.
As you get back on profile and speed, the plane will return the THR DES and all will be happy in the world.
If you have to use a hard-selected speed due to ATC restrictions, sometimes the plane will cycle between THR DES, THR IDLE, and SPEED somewhat rapidly, as it cannot play with the glide to stay on profile. You have "closed its window" so to speak. Sometimes that can be a good time to use V/S for a smoother descent.
Basically THR DES gives the computer flexibility to accommodate changing conditions by both adding or reducing power if the "line" it plans doesn't work as it planned it to!