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 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.

Reiraku Users
All Shinigami and Quincy have access to the Reiraku technique. In fact it was first seen being used by Uryū Ishida, the last Quincy (though he really the second to last.)