Actually, i digged a little in our different manuals and found the following excerpt in our OM B III NG:
Autothrottle Engagement
Moving the A/T Arm switch to ARM, arms the A/T for engagement in the N1,
MCP SPD or FMC SPD mode. The A/T Arm switch is magnetically held at ARM
and releases to OFF when the A/T becomes disengaged.
A general summary of A/T mode engagement is as follows:
...
• alpha floor automatically engages the A/T when armed.