Question: If Manas is Mind, Buddhi is Intelligence, and Ahamkara is Ego, then what exactly is subconscious mind?
Not every memory stored in buddhi is directly accessible by manas. This is one aspect of what psychologists are referring to with the “subconsciousness” concept.
Another aspect is that the mind has patterns of operation,” mental habits,” as a result of doing things in certain ways. These are termed, “saṁskāra.”
Yet another aspect of the “subconscious mind” is the part of buddhi which functions without words. The vast majority of our thoughts take place with the medium of words, but buddhi can use any pattern (not just verbal ones) to process information. The connection and thoughts that don’t use words are more difficult to articulate and dissect in the normal realm, because in the normal waking realm we are very involved in using words. By not using words, these thought processes can sometimes be freed from some of the incorrect definitions and understandings we have of the words we use.
Question: Is there also an unconscious mind?
Unconsciousness is a state, not a thing. That state is termed, susupta-jñāna.
Question: Many a times I have observed that certain thoughts and emotions just pop up without any external stimuli, more so when I am sitting idle. Do they come from the subconscious mind?
These are the saṁskāra, the mind’s habitual patterns, asserting themselves.
Question: Many self-help books and speakers say that there is a great potential at the subconscious level, which can be tapped. Is this a valid statement or just some fad?
What they are really saying is that if we pay more attention to life, we can get more out of it and accomplish more. This is a pretty obvious thing, but they package it in some appealing way and sell it as a new self-help technique.
They are also saying that the thought processes that happen without words can sometimes make amazing connections and discoveries.
Vedic culture valued meditation very highly. Activating the potential they are talking about is just one of the beneficial side effects of even a little meditative practice.