If it's the same as the 757, FLCH engages in FLCH SPD. The pitch controls the speed, the AT does some clever maths and works out whether it needs a bootload of thrust (for a big climb) or a tiny bit (for a 1000' climb etc.).
If you move the throttle, the mode changes to THRHLD SPD, and the AT "lets go" of the thrust levers. You are then (as mentioned) controlling the ROC/ROD
NW3