Reiraku

are the visualization of Reiryoku into threads which a spiritually aware person can follow. According to Rukia Kuchiki, this is a high-level technique. To the user of the technique, a Shinigami's spirit thread appears to be red, while that of other beings appear to be white. Ichigo Kurosaki used this technique 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 find his Shinigami powers, which were hidden in one box among millions, the boxes representing the humanity of his soul.

Reiraku Users
All Shinigami and Quincy have access to the Reiraku technique. How ever, the only two people ever being seen using is were Ichigo and Uryuu.