The effect Zeigarnik indicates the tendency to better remembering a task than it was carried out if this one were stopped. The fact of beginning in the realization of a task creates a motivation of completion which would remain dissatisfied if the task is stopped. Under the effect of this motivation a stopped task must be memorized better than a completed task.

Bluma Zeigarnik required of children to achieve, in one day, a series of twenty small work (to model animals, to thread pearls, to assemble the parts of a puzzle,…) The half of the activities could be finished, the others remained unfinished. Some time after, the participants were requested to indicate all the tasks which they had had to carry out. It in result that those which could not have been led to their term were quoted approximately twice more often only the others, as if the incompletion of an activity undertaken created a durable tension of the organization, whose memory would be only the print. Indeed, when one gives to the subjects the possibility of completing their work, it occurs a relaxation on their premises, and there is no more difference in memorizing between the accomplished tasks.

Zeigarnik, B. (1927). Das Behalten erledigter und unerledigter Handlungen. Psychologische Forschung , 9,1-85.