Zen instruction devotes an unusual proportion of its attention to the body, and the reason is doctrinal rather than merely practical.
The classical posture is full lotus, each foot on the opposite thigh, with half lotus, the Burmese position and a kneeling bench accepted as adaptations. The pelvis is tilted forward on a firm cushion so that the spine stacks without muscular effort; the ears align over the shoulders and the nose over the navel; the hands rest in the lap left over right with the thumbs lightly touching to form an oval, which functions as a feedback device, since the thumbs part when attention slackens and press together when it strains. The eyes are left open and lowered, unfocused, toward the floor or a blank wall. Closing them is discouraged as an invitation to reverie.
Soto practice faces the wall; Rinzai practice faces into the room. A period runs commonly thirty to forty minutes and is marked by a bell, with slow formal walking between periods, and during a sesshin these alternate from before dawn until night for a week at a time.
The doctrinal point is Dogen's. He argued that sitting is not a means to awakening but its expression: the practice is not a technique for producing a result later but the result already being enacted now, so that a person sitting is a buddha sitting, whether or not anything is noticed at the time. That is why the physical detail matters so much in this tradition and comparatively little in others. If the sitting is itself the thing, then the sitting has to be done properly.