Reiraku

are the visualization of Spiritual Energy into threads which a spiritually aware person can follow. Rukia Kuchiki says this is a high-level technique. Shinigami have a red spirit thread, while other beings have white. Ichigo Kurosaki was able to see spirit threads for the first time when searching for a Plus hiding inside a pet bird, and later when he had to journey to his "inner world" (the depths of his soul) in order to learn the name of his Zanpakutō.