Quote from the Boeing 777 manual
For those unfamiliar with the Autothrottle system on the B777, here's a quote from the Ops manual:
"With the autothrottle armed, the autothrottle automatically activates if:
• no autopilot or flightdirector active, or
• an autopilot or flightdirector is in VNAVXXX, FPA, ALT, V/S, or G/S,
and:
• speed less than an FMC calculated value for one second
• thrust below reference thrust
• airplane altitude above 100feet RA on approach, or airplane barometric altitude 400 feet above airport on takeoff.
Note: During a descent in VNAV SPD, the autothrottle may activate in HOLD mode and will not support stall protection".