PJ2... You really are making me get back in the books here
If the pilot pulls back slightly on the stick, the speed will decrease and continue to do so until reaching a speed just above the "lowest selectable" speed, (Vls in the graphic) at which time the descent mode will change from Idle-Open Descent to Vertical Speed and the autothrust reverts to "SPEED". This "mode reversion" occurs in other autothrust-manual flight circumstances as part of the protections under "Normal (fbw) Law".
The Reversion you quote is correct for older A320 series aircraft... newer or modified ones just turn the FDs off and go to SPEED.
What I do disagree with is
This "mode reversion" occurs in other autothrust-manual flight circumstances as part of the protections under "Normal (fbw) Law".
IMHO this is nothing to do with FBW... It can occur in any law in which FDs are available, and is part of the FMS (Flight Management / Guidance System).
...and this is an important point because in "IDLE - OPEN DESCENT" mode, the airplane does not operate like a Boeing
My "recollection" (which maybe wrong) is that (Idle) OP DES is pretty much like FLCH is a B757/767...
NoD