The Zeigarnik effect is the tendency of unfinished tasks (or open loops, as is commonly phrased) to hang around in working- or short-term memory until the task is completed (until we close the loop). This effect is a key observation behind the Getting Things Done system, and is detailed in Sönke Ahrens’ How to Take Smart Notes.
This observation captures why having too many tasks in progress can feel so very overwhelming: you only get Seven, plus or minus two slots for working memory, even having 2 open loops occupying that space puts a meaningful dent in your working memory capacity, and Working memory is a performance-limiting factor.
Fortunately, it seems that having a reliable plan eliminates most/all of the cognitive burden associated with the Zeigarnik effect, hence the utility of GTD and the reason that Systems for thinking must be comprehensive and reliable.
References
Ahrens, Sönke. How to Take Smart Notes