It's based on dianetics and engrams, which are parts of Jungian depth psychology.  An engram is what McLuhan would call a "cliche".  Something you've heard over and over so it's lost its energy.  But it's buried in the psyche.

Engrams can be worked on, for example, by brushing it up against another engram in a kind of chemical reaction of the psyche (cliche to archetype).  Consider the banal words "red" or "bull" evoke no reaction.  But together they release energy.  It's a kind of chemisty of the individual psyche.  But can apply quite well to the collective (think psy ops).

What scientology does is something called "dianetics" which puts you in a semi-trance like state (like NLP), and then investigates your psyche and engrams.  The simple tool is to find the first psychological barrier that is blocking your and see if you can unblock yourself on that.  Rinse.  Repeat.  This is a bit like some yogic techniques like the Kundaliní first chakra.

All well and good, however Hubbard also figured out you could do more by making a sci fi myth around it, and try and get people to believe it.  That's the start of problems.  Secondly, they are something of a racket.  They target celebs and then stalk them.  Trying to extract the maximum cash from them.  They infiltrate large organizations, inc. the US Coast Gaurd.  They steal rubbish and go through it trying to find out how tney can black mail famous people.  Very little known story was they stalked Led Zepplin "I hear a rustle in the hedgrow".  The BUYING a stairway to heaven was a direct jibe against those guys.

I guess I know more about this than I should, lol.  I think there's some valid psychological techniques there, but you can just do it yourself.  He basically tried to start a mythology, and it devolved into an anticult, which was, of course, inevitable.  As with alot of things, it will work for some people, but probably not for everyone.