The Rolls Royce Merlin engine was equipped with an 'SU' carburettor. When the aeroplane performed a negative g force manoeuvre (pitching the nose hard down), fuel was forced upwards to the top of the float chamber, depriving the engine of fuel. This would produce a momentary loss of power. If the negative g continued, the excessive fuel would force the float to the floor of the float chamber, opening the needle valve and flooding the carburettor with fuel and drowning the supercharger with over-rich mixture. This would lead to a rich mixture cut-out, which would shut down the engine completely, which was a serious drawback in combat.
Negative g commonly occurs when manoeuvring to fire on an enemy aircraft. Moving the stick forward would starve the engine of fuel, produce a sudden loss of power. This would let the enemy get away, and if continued even flood the engine. The German fighter pilots, whose aeroplane engines were fuel injected and therefore did not suffer from this problem, could exploit this by pitching steeply forward, a manoeuvre the British fighters could not follow. The British countermeasure, a half roll so the aircraft would only be subjected to positive g as they followed German aircraft into a dive, took enough time to let the enemy escape in most instances.
While not solving the problem fully, the restrictor, along with modifications to the needle valve, permitted pilots to perform quick negative g manoeuvres without loss of engine power, removing the annoying drawback the British fighters had had in comparison to the German Messerschmitt Bf 109 machine, which was equipped with fuel injection. Miss Shilling with a small team travelled around the countryside in early 1941 fitting the restrictors, giving priority to front-line units. By March 1941 the device had been installed throughout Fighter Command. Officially named the 'Miss Tilly's Diaphragm', the restrictor was immensely popular with pilots, who affectionately named it 'Miss Shilling's orifice' or simply the 'Tilly orifice'.
This simple yet elegant solution was only a stopgap: it did not permit an aeroplane to fly inverted for any length of time. The problems were not finally overcome until the introduction of Bendix and later Rolls Royce pressure carburetors in 1943.