MAYDAY MAYDAY, to clarify your understanding the aircraft can still manage the thrust to descend to its profile at the particular speed you set, letting it have 'managed speed' just gives it more flexibiility for on the spot profile maintenance. Even in managed speed it intends to keep to the speed that is central to its allowed range, e.g if the economy descent speed is a typical 300 kts, it can (if it needs to) descend at 320, or slow to 280 depending on other factors, e.g. being held high/stronger than forecast wind etc.
Further to that modifying the speed in the CLB/DES pages is only really useful prior to departure/in the cruise respectively - more of a planning feature. Speed management in the climb/descent is done through the FCU(flight control unit - speed selection window).
hope that helps - and makes sense!
