Hara hachi bu is the Okinawan cultural habit of eating only until you are 80% full. Researchers believe this dietary habit coupled with the traditional plant-based diet plays a role in the longevity and good health of the Okinawan people.

Stopping at 80% capacity is a good strategy to avoid weight gain without going hungry because the stomach's stretch receptors take about 20 minutes to tell the body that how full it really is.  

References

Suzuki M, Wilcox BJ, Wilcox CD.Implications from and for food cultures for cardiovascular disease: longevity.Asia Pac J Clin Nutr. 2001;10(2):165-71. Sho H.History and characteristics of Okinawan longevity food.Asia Pac J Clin Nutr. 2001;10(2):159-64. Yamori Y, Miura A, Taira K.Implications from and for food cultures for cardiovascular diseases: Japanese food, particularly Okinawan diets.Asia Pac J Clin Nutr. 2001;10(2):144-5.