From FCOM 1.22.30 p65:
'The RETARD mode is only available during automatic landing (AP engaged in LAND mode). At approximately 40' RA, the RETARD mode engages and remains engaged after touchdown. The A/THR commands IDLE thrust during the flare, and the FMA and enginge warning display IDLE. If the autopilot is disengaged during the flare before touchdown, the speed mode replaces the RETARD mode and the plot has to manually reduce thrust.
Note: In an automatic landing, the system generats a RETARD callout at 10' RA which prompts the pilot to move the thrust levers to IDLE in order to confirm thrust reduction. In a manual landing conditions, the system generates this callout at 20' RA as a reminder.'