The seminal text about this diagram from In Search of the Miraculous, P.D. Ouspensky's famous book about his work with Gurdjieff, is below:
"G. returned to the enneagram many times and in various connections.
“Each completed whole, each cosmos, each organism, each plant, is an enneagram,” he said. “But not each of these enneagrams has an inner triangle. The inner triangle stands for the presence of higher elements, according to the scale of ‘hydrogens,’ in a given organism. This inner triangle is possessed by such plants, for example, as hemp, poppy, hops, tea, coffee, tobacco, and many other plants which play a definite role in the life of man. The study of these plants can reveal much for us in regard to the enneagram.
“Speaking in general it must be understood that the enneagram is a universal symbol. All knowledge can be included in the enneagram and with the help of the enneagram it can be interpreted. And in this connection only what a man is able to put into the enneagram does he actually know, that is, understand. What he cannot put into the enneagram he does not understand. For the man who is able to make use of it, the enneagram makes books and libraries entirely unnecessary. Everything can be included and read in the enneagram. A man may be quite alone in the desert and he can trace the enneagram in the sand and in it read the eternal laws of the universe. And every time he can learn something new, something he did not know before.
“If two men who have been in different schools meet, they will draw the enneagram and with its help they will be able at once to establish which of them knows more and which, consequently, stands upon which step, that is to say, which is the elder, which is the teacher and which the pupil. The enneagram is the fundamental hieroglyph of a universal language which has as many different meanings as there are levels of men.
“The enneagram is perpetual motion, the same perpetual motion that men have sought since the remotest antiquity and could never find. And it is clear why they could not find perpetual motion. They sought outside themselves that which was within them; and they attempted to construct perpetual motion as a machine is constructed, whereas real perpetual motion is a part of another perpetual motion and cannot be created apart from it. The enneagram is a schematic diagram of perpetual motion, that is, of a machine of eternal movement. But of course it is necessary to know how to read this diagram. The understanding of this symbol and the ability to make use of it give man very great power. It is perpetual motion and it is also the philosopher’s stone of the alchemists."
Excerpt taken from In Search of the Miraculous by P. D. Ouspensky, pub. Paul H. Crompton Ltd, 2004, pp 293–294.