QA,
as far as I can tell from the requirements for EPR trimming it will happen on the ground as the lock-up occurs at 65 kts.
The unlock has four conditions,
airspeed over 300 kts.,
altitude over 4500' above runway,
altitude over 400' and throttle moved by more than 3 deg. or
altitude over 400' and FMC commanded unlock.
That's looking at it from the EEC point of view, what happens to the throttles in the cockpit I don't know.
VnV...