Songyuan asked,
Why can't clear-eyed Bodhisattvas sever the red thread?

The idea here is that the red theme runs through everyone's life. This read thread is passion and sorrow — all the vulnerabilty and desire that link you to the world. The direction this thread takes in your life is only gradually observable over time. It is the color of blood, of fire, of sex, of intimacy. To connect, to help, to be be of use in this world, you have to walk with people. You have to let them act upon you also, and you won't remain unchanged. . . Songyuan, who made this koan, was explicit about this. Sometimes he said, "it's the red thread between the legs."

John Tarrant, Bring Me the Rhinoceros