Descent of the Potency
A concept borrowed from Rudolf Steiner, the descent of the potency refers to an aspect of interiorization in which can be observed the following sequence: "the descent of immaterial forces into the material, which they create by so descending, followed by their setting free and re-ascent . . ." (UV 46).

As the Meggid goes on to explain, "It is again the same in the life and growth of any individual human being. The individual--shall I say the 'potency'?--which descends into the embryo and begins its work of informing, finishes the different stages of its work at different times during his life and then is progressively set free . . ." (UV 46).1

It was Steiner's contention that the informing forces are, in a sense, homologous throughout nature, so that there exists a real connection between, for example, the tendency of a river to flow along a convoluted path and the convolutions of the human brain. (See, for example, the Steiner-inspired Sensitive Chaos: Flowing Forms in Water, Wind and Air, by Theodore Schwenk.)

The descent of the potency is perhaps best thought of as "a sort of objective geometrizing, which is engaged in giving the human form its whole shape and quality--and to the brain the configuration which will be required to support it when it later blossoms as conscious activity" (WA 142; Sanderson is speaking).

The descent of the potency is not an invisible process. As the Meggid points out, it is

really not at all difficult to see that "interior" forces, which have been producing a physical form by incarnating, are set free when their work is completed. It happens . . . over much of the earth every autumn. When the stem and the leaves, and at last the flower, have been given physical form, we glimpse their departure in the revelation of light and colour and scent which ascend from the blossom like a cloud, on their way back to immateriality. You will see also the difference between the only way of thinking that was open to the Gabriel age--and the way which is open to the age which is now upon us. (UV 42)
The descent of the potency is operative in the entire biological world, as Barfield reminds in Worlds Apart:
If you really look . . . at a simple phenomenon like, say, a flower and the butterfly hovering over it, it will really tell you of itself that it is not something that arose through the interplay of discrete physical units, but the material manifestation of an immaterial unity from which both flower and butterfly have sprung. An invisible "common ancestor," if you like. (157-58, Sanderson is speaking)
See in particular Unancestral Voice, passim.
1As the Meggid explains, the acquistion of our adult teeth marks one such phase familiar to everyone:
    The earlier stages of release-I mean of course, the release of the informing forces-are evinced by easily perceptible and outwardly physical signs. . . . The physical process of second dentition is the product of the first release. Before that stage in childhood is reached, mind and body are one in a way that the man will never know again. (UV 46)
The coming of puberty is another:
    When he [a young man] reaches the age of puberty, it is because the informing forces have completed a further stage of their work and are correspondingly released. The man experiences this, in part, as a new independence of mind. Before, he imitated; now he is ready to pass judgment on his environment. But he also experiences the potency direct-with a new intensity-and more nearly in its own nature, as potency-now that it is continuing to work only in part of his being instead of through the whole. Mind and body begin trying to go their separate ways. He is tormented in one way or another by the gulf between them. (UV 46-47)