Remember what FLC (VFLCH) does--speed control using pitch. It controls pitch with the elevators followed by the necessary trim. Power is assumed to be constant, either idle or climb power, depending on whether going up or down. If airspeed at engagement is very different than that programmed, up or down goes the plane. If airspeed is varying, due to turbulence or atmospheric conditions (temp, being primary one) the autopilot will use pitch to maintain the speed.

GF