When a basketball player sees a teammate out of the corner of his eye, the player is using his peripheral vision. You can increase your ability to see action to the side without turning your head.
“Ninety-seven percent of vision is peripheral. In many sports situations, especially team sports, processing of information from peripheral vision is an essential element of successful performance. Peripheral vision is critical to generate accurate eye movements, maintain optimal balance and guide the best motor responses.”
Graham Erickson, O.D., FAAO, FCOVD
Professor at Pacific University College of Optometry, Author of Sports Vision: Vision Care for the Enhancement of Sports Performance