The fundamental interaction that is sometimes symbolized by the yin/yang symbol is the binding action of Prakrti (Cosmic Operative Principle) on Puruśa (Cosmic Cognitive Principle). This binding action is the fundamental interaction.
The binding action of Prakrti on Puruśa can also be described as “subjectivation” where the resultant expression is termed “objectivation”. Subjectivation is always followed by Objectivation.
Please see the next section for authoritative explanations of the Puruśa/Prakrti (Shiva/Shakti) terms by Shrii Shrii Anandamurti.
Transformation of Being to Becoming proceeds by 1 becoming 2 and 2 becoming 3. The dualities and multiplicities emerge from the 3.
The number 1 symbolizes the Unity of Being. The number 2 symbolizes Puruśa/Prakrti (Shiva/Shakti) aspects of the Unity of Being. Duality is the outcome of the fundamental interaction which can only be described by 3 gunas (tendencies). The number 3 symbolizes the 3 gunas. The “objectivation” (Creation) proceeds with the interplay of 3 gunas creating multiplicities.
The 2 opposing tentencies tamoguna and sattvaguna are mediated by the rajoguna. I listed other aspects of the 3 gunas in the article titled “Aspects of Rajoguna, Tamoguna, Sattvaguna” and proposed a mathematical expression for the conservation law of 3 gunas.
Shrii Shrii Anandamurti’s explanations are very enlightening:
“Sometimes the word “Prakrti” is erroneously used for nature. We say “it is a natural creation”, but there is a difference between nature and Prakrti. Nature is the functional style of Prakrti, Prakrti is not nature.”
“The universe is also created by principles – the Cognitive Principle, Parama Puruśa and the Operative Principle – Paramá Prakrti. Though it sounds dualistic in theory it is monistic in its spirit. The composite name of Shiva and Shakti is Brahma.”
from “Lord and His Name” (1 August 1966), published in Subháśita Saḿgraha Part 18
“The Supreme Entity means the Supreme Cognitive Principle. The Cognition is the Supreme Entity and not the Operative Principle, because this Cognitive Principle is the Transcendental Entity and the Operative Principle or Creative Principle is functioning within the jurisdiction of Consciousness. This universe is the creation of the Operative Principle within the jurisdiction, within the campus of the Transcendental Entity, of the Cognitive Principle. When it operates, the Operative Principle creates something concrete i.e. the Consciousness is transmitted into abstract and abstract is metamorphosed into the quinquelemental entity, then that Operative Principle is called the Creative Principle. In sanskrit we call the Creative Principle “Máyá”, and the functional attributes, the collection of the functional attributes of this Creative principle is nature.”
from “Triangle of Forces and the Supreme Entity” (17 November 1967), published in Subháśita Saḿgraha Part 19
“Brahma is the composite of Puruśa and Prakrti. Prakrti is the Operative Principle and Puruśa is the Consciousness or the Cognitive Principle. Brahma is two in one. Jiiva or the unit manifestation of Brahma is also composed of Puruśa and Prakrti, that is cognitive and operative principles. The sense of ego or “Doer I” that exists behind all that a person does, says, thinks, understands, or hears is the product of Prakrti. And behind this ego, the “I know I am” that exists as the Supreme Knower is a human being’s cognitive force – the undistorted Puruśa. Not only humans, every entity, whether animate or inanimate, movable or immovable, consists of Puruśa and Prakrti. Sentience or insentience are only relative conditions.”
“It is by the influence of Prakrti that Puruśa comes to be imbued with inertness, for to make the integral body of Brahma divided and diversified, everything has to be demarcated, and that demarcation is not possible unless the thing is crude. So to bring about such plurality in Puruśa, inertness has to be imposed on Him and this imposition is accomplished by Prakrti. Prakaroti iti Prakrti, that is, that, which [[has the]] multi-creative power or plasticity is called Prakrti.”
“This Prakrti has three constituent attributes or forces (guńas) – sentient (sattva), mutative (rajah) and static (tamah). Had there been no such multi-creative, triple-attributive Prakrti, the diverse manifestations of different animals and plants would not have come into being. In other words the entire universe would have remained purposeless and unvaried, unexpressed or unmanifested.”
“Prakrti and Máyá are almost synonymous, but in common usage there is a shade of difference in the meaning. In cases where the three guńas or attributes, sattva (sentient), rajah (mutative) and tamah (static), are in equilibrium, the word Prakrti, is generally used. Such equilibrated Mulá Prakrti (Primordial Force or Operative Principle) is unmanifest but where the three components are not in equilibrium, such a state is called Máyá (the Creative Principle). Where there is no equipoise or equality, where there is disparity – either sattva or tamah is predominant. The state of disparity in which sattva (the Sentient Force) is greater, is called Vidyámáyá or Introversive Force and the state in which Tamah (the Static Force) is greater, is called Avidyámáyá or Extroversive Force.“
from “The Intuitional Science of the Vedas – 6″ (1956), published in Subháśita Saḿgraha Part 3
“The term shakti has various meanings and as such has no proper English equivalent. “Energy” is the term generally used in English, but it does not convey the full import of shakti. The sentient, mutative and static principles of Prakrti are called shakti (Prakrti is called Prákrta Shakti or Supreme Operative Principle.). The way in which Prakrti functions is called shakti. The flow of the tanmatras or inferences is called shakti. The luminous factor is called shakti in a limited sense. And the energy which a living body acquires from its surroundings is called shakti. This is the broad range of meanings that the term shakti encompasses. Puruśa is also a kind of shakti – Citi Shakti or Cognitive Faculty.
Let us see how material energy originates. What is the relationship between energy in the mundane sense and the energy of the three principles? Although the sentient principle influences Puruśa (Cognitive Faculty) to some extent, it cannot bring Puruśa within the limitations of bondage. The influence of the mutative principle arouses the feeling of authorship in the Cognitive Faculty, but it cannot bring Puruśa within the scope of objectivization and demarcation. Objectivization and demarcation can only occur under the influence of the static principle. The Cognitive Faculty comes within the scope of bondage (theoretically) due to sentient influence; comes within the bondage of authorship due to mutative influence; and assumes the shape of certain demarcated forms due to static influence.
When form remains in the abstract realm it is called energy; when it comes into the material realm it is called matter. Energy is not one of the fundamental factors, but a stage prior to the metamorphosis of the Cosmic Citta into the five fundamental factors.
Expression is the innate characteristic of shakti. The sentient, mutative and static principles are born at a particular time but lie latent within shakti. They are the creators of the mind. It would be futile for the unit mind to attempt to trace the origin of these three principles because in the process it would land itself in the fallacy of infinite regress. When microcosms become active the unborn Prakrti is not manifested in them spontaneously. Microcosms have to draw the impetus to arouse Prakrti’s principles from the Macrocosm. Matter receives its impetus from both microcosm and Macrocosm.
Expression of energy is systaltic. And in this systaltic expression the three principles have their respective roles to play. The initial expression of a wave is sentient. When it reaches maturity it becomes mutative. The period of maturity is followed by a period of contraction. When the most contracted stage is reached the mutative principle is finally superseded by the static principle. This stage of maximum contraction contains the seed for the subsequent stages of expression. It marks the completion of one phase in the cycle of actional flow. At this point the microcosm gathers momentum for the next stage: the gradual unfoldment of microcosmic mind.“
from “Shakti and Its Proper Application” (19 December 1958), published in Ananda Marga Ideology and Way of Life in a Nutshell Part 9 [a compilation]
 Books of Shrii Shrii Anandamurti (Prabhat Ranjan Sarkar):