Thank you guys, that clears it up, from the link JL posted above --
The flight crew arms ATHR:
-On ground
- By pushing the ATHR pb on the FCU when the engines are not running, or
ATHR is active when it controls thrust or speed......The ATHR being disconnected, is activated when the flight crew pushes the ATHR pb on the FCU while the thrust levers are within the active range, including IDLE position. Note: When the flight crew sets both thrust levers to IDLE position, the A/THR disconnects but, if the flight crew pushes the A/THR pb of the FCU, they will simultaneously arm and activate the autothrust.