The historic use of your speed brake during the descent will have nothing to do with a new Vapp. And as other posters have mentioned what you are seeing is likely ground speed mini, designed to maintain a constant energy level, if you have a magenta speed target which reads above the Vapp visible in the FMGC, then this is almost certainly it.
Many years ago, and if I recall correctly airbus did have a feature (or maybe it was just my company as they had some quirky sop's) where you could activate 'phase advance'. Which meant clicking the thrust levers forward out of the climb detente for a split second in order to cause the aircraft thrust to increase... but that was removed and is no longer procedure.
I'd probably have a read about ground speed mini, because as other posters have said, by disabling it you are removing a safety feature.