What secret was Swami Vivekananda saying about in this phrase?

Question

At The Parliament of Religions in Chicago 1893, Swami Vivekananda spoke about Paper on Hinduism. A part of it contains this:

There is another suggestion. Taking all these for granted, how is it that I do not remember anything of my past life ? This can be easily explained. I am now speaking English. It is not my mother tongue, in fact no words of my mother tongue are now present in my consciousness; but let me try to bring them up, and they rush in. That shows that consciousness is only the surface of the mental ocean, and within its depths are stored up all our experiences. Try and struggle, they would come up and you would be conscious even of your past life.

This is direct and demonstrative evidence. Verification is the perfect proof of a theory, and here is the challenge thrown to the world by the Rishis. We have discovered the secret by which the very depths of the ocean of memory can be stirred up — try it and you would get a complete reminiscence of your past life.

What is this secret of revealing our past lives that Swamiji was talking about?

Answer 1

If you read more on the same page, you will come across this:

The human soul is eternal and immortal, perfect and infinite, and death means only a change of centre from one body to another. The present is determined by our past actions, and the future by the present. The soul will go on evolving up or reverting back from birth to birth and death to death. But here is another question: Is man a tiny boat in a tempest, raised one moment on the foamy crest of a billow and dashed down into a yawning chasm the next, rolling to and fro at the mercy of good and bad actions — a powerless, helpless wreck in an ever-raging, ever-rushing, uncompromising current of cause and effect; a little moth placed under the wheel of causation which rolls on crushing everything in its way and waits not for the widow's tears or the orphan's cry? The heart sinks at the idea, yet this is the law of Nature. Is there no hope? Is there no escape? — was the cry that went up from the bottom of the heart of despair. It reached the throne of mercy, and words of hope and consolation came down and inspired a Vedic sage, and he stood up before the world and in trumpet voice proclaimed the glad tidings: "Hear, ye children of immortal bliss! even ye that reside in higher spheres! I have found the Ancient One who is beyond all darkness, all delusion: knowing Him alone you shall be saved from death over again." "Children of immortal bliss" — what a sweet, what a hopeful name! Allow me to call you, brethren, by that sweet name — heirs of immortal bliss — yea, the Hindu refuses to call you sinners. Ye are the Children of God, the sharers of immortal bliss, holy and perfect beings. Ye divinities on earth — sinners! It is a sin to call a man so; it is a standing libel on human nature. Come up, O lions, and shake off the delusion that you are sheep; you are souls immortal, spirits free, blest and eternal; ye are not matter, ye are not bodies; matter is your servant, not you the servant of matter.

The secret Swami Vivekananda was referring to was the knowledge of Brahman.