The 30 Second Skinny Engrams are memories containing emotional charge. Thei removal of that charge is at the heart of both Dianetics and Scientology, but they were only ever a theoretical object in Hubbard's original hypothesis. Evidence has shown that they don't fit in with his original description for them, nor with our growing understanding of the way the human mind works.
What are Engrams?
According to the theory of Dianetics, an Engram is a memory containing an emotional charge. Unlike normal memories, Engrams are said to have the power to cause us to act irrationally. They "block" the Four Dynamics, stopping the Analytical mind from functioning correctly. L Ron Hubbard believed that the Reactive mind stored these engrams inside every cell of the human body. Unfortunatley Hubbard doesn't go into too much detail on the structures and mechanisms of these engrams, but we are able, from our current understanding, to infer various things about them.
If there is information stored in the cells themselves, then it must be stored physically. This information, incidentally, would consist of a perfect recording of all the senses along with whatever reactive mind processes that arouse from them. According to Hubbard these would be recorded to each and every cell in the body whenever we were partially or wholly unconscious.
Such a level of coding is not impossible. We are, after all, complex organisms whose shape is derived from a strand of DNA. In DNA we can find an example from nature of something that stores a great deal of information in a very small space. The decoding of that information is a lengthy process - 9 months - 70+ years depending on how you look at it. However, the engram must be much more sophisticated than DNA. We're looking at something that could encode, record, index, retrieve and decode a huge amount of information more or less instantly. Not only that but these engrams must be passed on when a dying cell is replaced by a new one. To continue our DNA parallel, given that we are told engrams are perfect recordings, then the transfer of information must also be perfect. The method of duplicating DNA, though, is not perfect. It can go wrong, often with catastrophic effects. Can we expect the same from engramic duplication, given the speed and scale of it?
Another peculiarity about the notion of the engram is that it is much more efficient than normal memory. We know that memory is, with a great degree of certainty, a brain function, but if we have the ability to store information in every cell of our bodies then the brain's ability to record information across its many cells becomes comparitively inefficient, especially considering that those cells are, according to Dianetics, capable of storing memory. It would be like taking a large number of laptops and using them as the beads in a giant abacus. When we consider the amount of energy and evolutionary time in getting something like a brain together then the idea that much of its function is redundant or at best overdesigned seems ridiculous.
Further comparison to normal memory is appropriate here, too. Our brain is fed information by way of the nervous system. We can think of this nervous system as something like a tree, with the brain at the base of the trunk and the pathways leading off to the various perceptors throughout the human body. If we think in these terms we should see at once that most perception happens very close to the brain. The eyes (sight), ears (hearing), nose (smell), tongue (taste) are all "head mounted". The main exception to this in the five main senses is touch, which by its very nature cannot take place in the head. It seems, then, that evolution has brought about an organism that benefits from the efficiency of having the information gathering happen as close as possible to the organ set to process and store it. If we think primarily in terms of memory, all the information being gathered is channeled in one direction, towards the brain. But engrams rely on these same perceptions, and engrams are stored in every single cell of the human body. A nervous system that supports such a process would be hideously complex.
One might argue that if the nervous system is there, then this information can be channelled throughout the body with ease, but this is unlikely. These nerve paths all perform specific functions. The optic nerve, as an example, relays information about vision. It is clear from the anatomy of the optic nerve that it is given over specifically to a single function - again if engrams are stored in every cell of the human body, and to that end any nerve will do to transmit all of the information required, then such tailoring to specific functions would seem unnecessary.
Why Do We Care About Engrams Anyway?
Engrams are key to the spiritual development of people practicing Dianetics and Scientology. Auditing, on or off the e-meter, consists of tracking down these engrams in our "track" or memory and removing their emotional charge. It's much like modern NLP techniques where memories are revisited but "rendered" in a way as to belittle them, and so weaken whatever impact they have on us. If we believe a cliff-top visit as a child has left us with a debilitating fear of heights, then we would revisit that memory but perhaps imagine it as a silent comedy, or perhaps even just as a smaller, less clear image. In auditing the moment is described over and over again until the engram no longer has an emotional charge. When a Scientologist describes someone as being "clear" it means that they have, through auditin, systematically removed all the emotional charge from all of these engrams. Once an engram is cleared it is refiled in normal memory. That's the theory any way.
The trouble is what occurs when someone is taken back to an incident that took place while they were still in the womb but even then the engram cannot be discharged. Where do we go from here? It is this occurence in auditing that is at the root of Scientology's belief in reincarnation. Even in the womb, the subject is instructed to go back to an earlier similar incident. These incidents can only happen in previous lives, and so in a transparent bit of leading, the auditor invites the subject to recall incidents from past lives.
Which takes us back to the notion of the engram as a cellular memory. If this is the case, then how are the engrams transferred from past lives to our current life? Suddenly our engram bank not only holds a recording of everything that happens to us when we're unconscious, but also of every unconscious moment of every previous life. The belief in past lives effectively knocks a hole into the key concept at the heart of Scientology.