Normally, either pitch AP/FD mode or A/THR will acquire and maintain the speed target. When thrust is fixed, pitch keeps speed. Thrust keeps speed when pitch keeps something else (V/S, G/S).
However, in a managed DES speed is allowed to deviate from target (within a speed band) while thrust is fixed (THR IDLE or THR DES) and pitch maintains the target flight path, which is the computed descent profile.
While inside the speed margin above and below the managed speed target, neither pitch nor thrust care about speed. Only whe pn speed hits a the boundary things change:
If speed reaches upper limit, then pitch will maintain that speed and no longer the profile.
If the speed reaches the lower limit, then thrust will maintain SPEED at the limit, instead of THR IDLE, while pitch will keep maintaining the profile.
In between those cases, speed will "float"