Śaṅkara is entirely on common ground with the Sūtras when he defends upanishadic monism by declaring not only that the effect is real before its production, but also that it is real before and during manifestation, and even that its future existence is real now and subject to apprehension by a yogin.
It is real, however, not in itself, but only as the cause, as the pot is real not qua pot but qua clay. The doctrine of the Sūtras that the objects come forth from, and return back to, the Absolute is defended.
But at times Śaṅkara goes back behind the doctrine of the Sūtras to certain texts of the old Upanishads and maintains that the effect is strictly nothing over and above the cause. As we already know, if the cause is said to be identical with the effect, this means that the effect has the nature of the cause, while the cause does not have the nature of the effect.
This, however, is to reduce the effect to the cause. If the world has an intelligible structure, and curds can only be obtained from milk and not from clay, this means that a certain power or predisposition to evolve into curds must be in the milk.
But Śaṅkara declares that all objects are non-different from the ‘powers, (śakti) or predispositions from which they proceed, while the powers, in turn, are non-different from the substances in which they lie, and the substances themselves are traceable finally to the great elements from which they proceed and into which they will eventually dissolve back, so that the whole world, beginning with the primordial element ether, is reduced to a mere inexplicable appearance arising on the face of the Absolute, while causality, law and intelligibility still reign within the appearance.
Śaṅkara transforms the older doctrine of Sat-kārya Vāda, the mere doctrine of the reality of the effect before its manifestation, into an instrument for affirming, yet again, the transcendence of the Absolute, which triumphs ultimately over all predicates attributed to it by the human mind.
~A J Alston from 'Shankara on the Absolute', p.126