“Opening from below” means this—that the supramental force descending awakes a response from below in the earth consciousness so that it is possible for a supramental activity to be formed in the material itself. All is involved as potentiality in the earth consciousness—life, mind, supermind—but it is only when Life Force descended from the life plane into the material that active and conscious organised life was possible—so it was only when mind descended that the latent mind in Matter awoke and could be organised. The supramental descent must create the same kind of opening from below so that a supramental consciousness can be organised in the material.
Ref: Letters on Himself and The Ashram