In order to maintain a person’s flow experience, the activity needs to reach a balance between the challenges of the activity and the abilities of the participant. If the challenge is higher than the ability, the activity becomes overwhelming and generates anxiety. If the challenge is lower than the ability, it provokes boredom. Fortunately, human beings have tolerance, there is a fuzzy safe zone where the activity is not too challenging or too boring, and psychic entropies like anxiety and boredom would not occur.

Due to the special relationship between challenge and ability, Flow has been used in fields like sports and tutoring. The famous GRE test is a good example of design based on the concept of the Flow Zone.

To create an immersion environment, it needs the balance between ability (A) and challenge (C). From the user’s activities and feedbacks, the system measures the relationship between A index and C index, which is basically the slope of the fuzzy zone. Then dynamically and automatically, the system adjusts the C index based on the A index to keep the user in the flow.