Leo is credited as being one of the most important astrologers in the 20th century because it appears that his work had the effect of stimulating a revival of astrology in the west after it was dethroned towards the end of the Renaissance.
Leo was a devout Theosophist and he worked many of those concepts, such as karma and reincarnation, into his astrology.
He used the Theosophical Society’s vast international connections to publish, translate and disseminate his work across Europe and America and it was in these countries that astrology began to be revived.
Leo was somewhat discouraged very early on in his studies at the complexity of much of astrology and how inaccessible it was to the student. As a result of this he set out to simplify astrology drastically in order to make it easier to disseminate, learn, and practice. One example of this simplification was his teaching that the meanings of certain signs, houses and planets was all essentially very similar and interchangeable, almost to the point of being the same thing or having the same meaning.
Leo also started the movement towards a more psychological astrology because he was the first astrologer to really direct the focus more towards character interpretation instead of straight prediction.
Leo travelled to India at one point in his life where he studied Vedic astrology for a period of time. He later attempted to incorporate a large portion of this Vedic astrology into western astrology, and although this synthesis of the two traditions never fully caught on in the west, there were a few specific techniques that were picked up by later astrologers such as the decanate and the dwadashamsha.
Sources
Nicholas Campion, Astrology History and Apocalypse, Centre For Psychological Astrology Press, London, England, 2000, ISBN 1-900869-15-4
James Holden, A History of Horoscopic Astrology, American Federation of Astrologers, Tempe, AZ, 1996. ISBN 0-86690-463-8
Alan Leo, Esoteric Astrology, Destiny Books, Rochester, Vermont, 1989.