In a given synchronistic experience, the subject perceives a coincidence in the external world; attached to this experience is an impression of meaningfulness, even a “numinous” feeling. To undergo an experience like that is not deliberate: the subject has not consciously chosen to be aware of the coincidental events, nor of the accompanying feeling of meaningfulness.
Such an episode is then an instance of an unconsciously driven perception; a synchronistic experience is a sub-type of a more general type of experience where any conscious act may be driven unconsciously (similar to compulsive behavior, unbidden memories, sudden appearance of emotion, feelings of deja vu, …). What singles out the experience of synchronicities is that it is specifically a perception of coincidental external events.
In this, it is the converse of compulsive behavior or disruptions of action (including speech action). When something drives us to check whether the stove is turned off, again and again; when we find ourselves slowing down or speeding up our steps against our will; or when slips, hesitations, inability to remember names creeps into our talk — the unconscious intervenes in processes with person-world direction (the person’s will and mind produce effects in the external world, i.e. actions and speech). Conversely, when we suddenly perceive notable coincidences in what happens, the unconscious forces itself into a world-person directed stream: perception and understanding our surroundings. Unconsicous break-ins with that direction seem to be a rare kind, however; most others have person-world direction (which makes it more obvious to interpret them psychologically).