Conclusion
It may be noted that although there is no material  difference between Raja Yoga as explained here and as found in the Yoga Sutras of Patanjali in so far as they relate to the final goal of Realization, there  is a difference between them in the practices to be adopted for reaching that  goal. Patanjali has prescribed control of the body and prana prior to  the practice of meditation whereas Sankara here emphasizes the meditation on  Brahman from the very beginning and thus aims at leading the aspirant directly  to the goal. Hence the title of the text is aparokshanubhuti.

The cause is concurrent and inherent in the effect but  not vice versa. Thus in the absence of the effect, the cause as such  disappears since there is nothing with reference to which it may be called as a  cause. When we identify with the world (effect) it is ignorance. Having crossed  ignorance when we identify ourselves with cause only (Brahman), the effect  (world) ceases to exist. When the effect has disappeared how can we call the  cause as a cause? So the idea of the cause also disappears. Thus the cause and  effect relationship comes to an end. This relationship is therefore a mere by  product brought about by the intellect. Thereafter only Pure Brahman (beyond  all relationships) remains which is beyond speech or description. This should  be kept in mind again and again recapitulating the illustration of the pot and  clay which says whether it is the cup or a jar or a pot they are only mere  words, just names but all of them are nothing but the cause, the clay.(Chandogya  Upanishad VI.i.4)

When both the cause and effect thus disappeared one is  apt to conclude that only Sunya or a void is left behind. But it is not  so. One may negate everything but cannot negate one’s own Self. So when  causality has been negated what is beyond all negation is the very Self of the  enquirer, which alone remains as the ultimate reality. Sankara concludes this  text with the statement that for those whose mind is completely purified alone  this Raja Yoga will be productive of perfection.

Epilogue: Dr.Radhakrishnan had remarked in a  different context that Sankara’s ideas are like darkness because of heavy and  powerful headlights. But humble beings like us can always take advantage of the  powerful headlights of Sankara and remove the darkness by adjusting their  voltage. This article is an attempt in that direction.

Reference:
1.Aparokshanubhuti or Self-Realization by Swami Vimuktananda
2.Aparokshanubhuti or Intimate Experience of the Reality by Swami Chinmayananda
This is a preliminary text book on Vedanta one should read  before plunging into the Upanishads.

Also read:
1.Mundaka Upanishad
2.Isha Upanishad
3.Suryopanishad
4.Kena Upanishad
5.Patanjali’s Yoga Sutras