The Zeigarnik Effect is based on the idea that it is human nature to finish what we start and, if we don’t finish something, we experience dissonance, resulting in an uncomfortable feeling. Not finishing something puts us in a state of tension that makes us pay more attention to the thing we want to finish. The consequence is that we remember uncompleted tasks more than the tasks already completed and are often driven by this effect to complete something. In other words, we have little motivation to recall things we've finished whereas we have a strong investment of interest in unfinished things and this keeps them in the forefront of our minds.

Russian psychologist Bluma Zeigarnik (after whom this effect is named) made note of this cognitive bias in a restaurant: she observed that waiters could remember complex food orders but once the table had received their food and paid their bill, all details would be forgotten. Orders of those tables that were still “incomplete” could be readily and easily called to mind (even if it had been some time since they’d taken their order or dealt with the table) whereas those of the “completed” tables were no longer in their memories. Once a table was completely taken care of then the details pertaining to that table would be “classed” as complete in the waiter’s mind and effectively erased from their memory in order to make way for new - or still relevant - information.

The Zeigarnik Effect has many possibilities for application in the commercial world. It is for example often used by TV shows or video games that engage the “cliffhanger effect” to keep watchers and users engaged with their product. It can also be utilised online to ensure customers don’t feel the sense of dissonance associated with this effect. Providing a clear and positive sense of progression and ultimate closure through displaying things such as progress bars can help to make your customer aware of how far along in the payment or sign-up process they have gotten and encourage them to continue on to completion.