Operation (Fig. 8A)

When the engine is started and the rotor turns within the sleeve, the rotor port at the control stop end becomes coincident with the port in the sleeve leading to the fuel reservoir in the body casting. Fuel at high pressure enters the rotor bore and drives the shuttle to the right (i.e., towards the fixed stop end of the rotor). This movement of the shuttle displaces fuel in the rotor bore through the ports in the rotor and sleeve to the injector serving No. 1 cylinder.

A further 180 degree rotation of the rotor (Fig. 8B) causes the rotor ports at the fixed stop end to align with the sleeve port leading to the fuel reservoir. Fuel now enters at the fixed stop end of the rotor and drives the shuttle back towards the control stop end. An identical quantity of fuel is displaced, by the shuttle as it moves to the left, by way of the outlets in the rotor and sleeve to No. 2  injector.
In this way the shuttle continually moves between the two stops displacing an accurate amount of fuel to each cylinder in turn.
The quantity of fuel delivered at each injection is dependent upon the distance the shuttle travels, this distance is adjusted by the control stop.
The control stop is moved via a "fuel cam".
Road race metering units have a simple "fuel cam" hooked directly to the throttle shaft with linkage. Production Triumph, Ferrari, Maserati etc., used a  complicated manifold vacuum "control unit" to control fuel cam position.. 

Fig. 8