Amaterasu (天照) is a referance to the japanese mythology's goddess of the sun, as Renji's technique is light based and gives him a light yellow aura when he uses it. The creation of this technique started when Renji started thinking that he was making poor use of his devil fruit and haven't released its full potential yet, he tried to think of a new way of using the light that he stores in his body rather then just just releasing it from his body. Thanks to his devil fruit, Renji is not harmed by this drastic rise of his body's temperature.