On the backs of the GD Tarot are three phrases- Khabs Am Pekht, Konx Om Pax and Light In Extension. Apparently these all mean the same thing in different languages- Egyptian, Greek and English respectively. On page 347 of The Golden Dawn it says: "Their significance implies, by various Qabalistic methods of analysis, as well as by certain reading of the Coptic and Egyptian heiroglyphics, 'Light in Extension' or 'May Light be extended in Abundance upon you.'" I couldn't find anything that said this was THE GD magical formula but to me it sums up their philosophy as good as anything.
Before Crowley, magic had been practiced the same way for centuries. The inner state of the magus was irrelevant; as long as certain formulas were followed in the prescribed manner and the correct words said, the heavens would open and shine its Light upon you. This was also the GD's approach. Crowley's formula was different; it placed the emphasis upon the subjective inner state of the magus. To me, this is one of Crowley's most important contributions, if not THE most.