When TO/GA is pressed and the levers advance to the requested N1 position,they can then be adjusted as they are then essentialy are in the ARM mode until throttle HOLD at 80kts.the ARM mode can also be seen in a VNAV descent for example,with VNAV PATH annunciated in the FMA the autothrottle will show ARM in white,you can then move the thrust levers to adjust aircraft speed (within +-15kts of requested speed.otherwise it reverts to FMC SPEED).
As far as i can recall this is what happens,but dont quote me on that!
cheers.