Why do flaperons 'move down then up' on takeoff roll on a 787?
I found this 16-year old thread on airliners.net. The Boeing 777 has the same system, and at the time it was unique in that. (You can watch it here.)

When takeoff power is applied, hydraulic power is removed from the flaperons, as not to stress the actuators for an extended duration, which causes them to drop due to gravity. They are then slowly lifted by the rushing air. At around 100 knots, power is restored to them.

The stress being avoided is from the engine's jet blast. During ground runs for an engine, the flaperon is known to flutter.



The poster cites the B777 Maintenance Training Manual Rev 4 02/08/1997. The reasoning comes from this other 12-year old thread.