In basic terms, the FPV should point in the direction the aircraft is travelling through the air (relative to the pitch and horizon indications on the Primary Flight Display).
So if you push forwards on the yoke, the aircraft will pitch down but initially the aircraft will still be travelling in the same direction due to its inertia, once the aircraft starts to descend in this example, the FPV indicator will start to move towards the 'Airplane symbol' on the PFD.