Is is basically an open descent or climb with greed dot or Vne as the speed target.
And A320ies don't derate climb, at least I have yet to see one which does.
The fact that this button has the opposite effect in A330/340 is something to stumble over if MFF flying both.
As is the target when pulling the speed knob (current speed on A320, current speed target on A330/340).