To be clear, I don't know anything about the 777!
But I do know that, in the 737NG if the autothrottle is armed during pre-flight checks, one only has to touch either of the TOGA switches and both engine thrust levers will immediately advance to takeoff power.
Thats why we don't arm the autothrottle until approaching the departure runway!
If the 777 system is anything like the 737 then . . . . . .
