To be fair the 'approved' method of stopping a Gipsy is to idle to cool the engine then earth the mags, whilst smartly advancing the throttle to fully open.
These engines are prone to running on and the cooling plus cold air (actually mixture) will usually stop the engine. However in hot conditions the engine does sometimes run on lumpily for up to 5-10 seconds - the mixture being ignited by hot deposits in the combustion chamber.
Really the key to this is earthing the mags. My O-200 has a Stromberg carb which does not have an ICO so it is only possible to stop it using the mag earth - so I handle my propeller with complete contempt!!!