When you’re doing shikantaza you don’t try to focus on anything specifically, or to make thoughts go away. You simply allow everything to be just the way it is. Thoughts come, thoughts go, and you simply watch them, you keep your awareness on them. It takes a lot of energy and persistence to sit shikantaza, to not get caught up in daydreaming. But little by little, thoughts begin to slow down, and finally they cease to arise. When the thought disappears, the thinker disappears. This is the samadhi of falling away of body and mind.
Whether we work on the breath, with a koan, or shikantaza, zazen eventually leads to samadhi. The first indication is usually an off-sensation of the body. This happens most frequently during sesshin because of the long periods of sitting. When you sit for a while without moving the body, it stops receiving information about its edges through the senses, such as the friction of your clothing or an itch on your leg. So although you know the body is there, you don’t feel it. Some people get frightened at this point, and involuntarily their body twitches and defines its edges. Then they slowly move to that place again; gradually they learn to trust it, and they begin to go a little bit further each time. Next comes the off-sensation of the mind. The mind is
dependent upon thoughts, but when the thoughts disappear the mind disappears, the self disappears. That constant reflex action that says, “I’m here, I’m here, I’m here” is the ego manifesting itself. This is when we realize that we are constantly re-creating ourselves.
Sometimes during sitting people have what we call makyo: a vision or hallucination. Other times it’s smells or sounds. Students often think this means they’re enlightened—particularly if the image is related to Zen, like the Buddha sitting on a golden lotus—and they immediately run off to dokusan to get it confirmed. The teacher will usually listen and then say something like, “Maybe you’re not sitting straight. Sit straight. Don’t worry, it will go away.” It doesn’t matter whether we attach to a regular thought, or to the thought of enlightenment. Whatever it is, it is still attachment.
– from Dharma Discourse by John Daido Loori, Roshi