If you fail to experience something that you intended to experience, then something interesting happens. Instead of just taking the learning, the flavor or essence of the experience, with you, you might take a snapshot of the whole incident with you for later processing.
As you go through life, you experience things and you learn from them. You go through a sequence of events that each has a start, a continuance, and an ending to them. If you go through the intended start, continuance, and ending of the event as planned, then everything is fine. The event is over with, whatever it was, and you drew the experience you needed out of it. It doesn't really matter if events were labeled "good" or "bad", if they were pleasant or unpleasant. As long as you get the desired experience out of them, they will not be aberrative in any way.
The trouble is with the incidents that weren't completely experienced because of information overload. Let's assume as a simplistic model that a person can process a certain quantity of experience per unit of time. If an amount of input below the limit is received, then she is probably comfortable with what is going on, and is learning from it. However, if the limit is exceeded, she gets overwhelmed with input.
When the processing powers of the mind get overloaded, it doesn't just ignore what is going on. The mind seems to have the ability to take complete snapshots of events and to store them for any length of time. The idea seems to be to process them "later".
If the overload is simply one of speed or magnitude of information, it is usually possible to catch up. For example, if you go to a lecture and you get a lot of new information in a short period of time. You might not be able to evaluate everything at the time, but after assimilating it for a couple of days you might be quite comfortable about having received the information.
But, if there is a content of the event that overloads your mental circuits in other ways, you might not process it at all. For example, if there is stronger force, or stronger emotions in the incident than you are willing to deal with, then you might never process the incident. For example, if somebody was hit by a car, they might not be able to process that kind of force, even very slowly, so the incident never gets processed. Or if somebody dies and is totally unwilling for that to happen, they might not process it.
Going through life, one handles most events fairly well. One notices what is happening, one gets wiser from having the experience, and all one carries forward in time is the added experience. Then an incident happens that is too much action in too little time. A whole facsimile of the event is taken for later processing. It is carried forward in its whole as a frozen incident.
The trouble is that the incident will continue insisting on being processed. It will pretend that it is still happening in the present moment, and it will present pieces of itself to the person for processing. But, if the majority of the incident remains overwhelming it might only present manageable little bits of the incident.
The person, not noticing that there is an unprocessed incident, might regard these little tidbits from the incident as current events and impulses. She might mistakenly think that they are happening now, and she might mistakenly act out parts of the incident believing them to be her instinctive responses in the present.
In other words, the unprocessed incident becomes an automatic program that gets replayed out of context. The mind will try to make the responses fit the situation at hand as well as possible, but it has difficulty doing so. The responses are likely to be inappropriate and are likely to get the person into some sort of trouble.
The whole problem is that the responses come from an unknown source. If the person doesn't realize the unprocessed incident is active and she just reacts automatically based on it, then her actions will be puzzling to both herself and others. If she knew the whole story consciously she would quickly realize that it is kind of silly and she would stop doing it.
An unprocessed incident might drop further away over time. If it continues being unprocessed and the environment or the intentions of the individual change to have less semblance to the original event, then it might by itself drop away and stop calling attention to itself. However, if the environment again starts being similar to some of the contents of the incident, then it might become re-stimulated and might again replay its contents automatically.
The conscious evaluation of events by the individual is needed in order to file experiences correctly. The individual needs to decide what is relevant and what is not. Until she makes such evaluations everything will be granted equal importance by the mind. So, everything in an unprocessed incident will be automatically considered equally important.
Since, for an unprocessed incident, EVERYTHING gets stored, including pain and force and emotions, it will get an importance that is way out of proportion. And aspects of it will be regarded as important that the individual would never consciously carry around if she had the choice. I.e. if the event was "being hit by a car", then the car's color might be just as important as the pain of the impact, which is just as important as being on the way to the supermarket.
When these elements are replayed later on, the results can be rather silly. The person might get a headache when going to the supermarket, or might start disliking red cars or some such thing. Really it is just the unprocessed incident trying to call attention to itself, but if nobody is noticing, it becomes some rather odd automatic reactions the person has to life.
So, in summary, any event that overloads the ability of an individual to process it, and that then gets recorded as an unprocessed facsimile, that remains unprocessed, floating through time, causing undesirable automatic reactions -- that is the target of Incident Clearing.