There are some critical distinctions to be noted between saññā-vedayita-nirodha, (i.e., the cessation of perception and sensation) and nirvana. Saññā-vedayita-nirodha is sometimes also refered to as nirodha-samāpati, the attainment of cessation.
Nirodha-samāpati, I should elaborate, is close to a kind of cataplectic, trance-like state which comes after a extended period of immobility such as cross legged sitting during zazen. It is not altogether uncommon during a sesshin. Once I was showing a friend of mine how to do zazen. After ten minutes (he was a beginner) I noticed that he was still sitting very still. So I kept sitting. I went on for maybe a half hour more. Then I got up—he was still sitting. Then I gently shook him. He had been in a kind of trance almost the whole time.
It has long been observed that when animals are put into a particular posture that they cease to struggle. I was first shown this by a friend of mine at his farm. He took one of his chickens and placing a piece of straw in front of the chicken, put the chicken’s beak on the straw, causing the chicken to freeze and stay with its beak on the straw for what seemed a long time.
Looking at nirvana, which is possible to attain by dhyāna (P., jhāna), although it is still independent of this exercise, it is a self-existing dharma (svabhāva-dharma). Not only is nirvana unconditioned, whether we realize it or not, it exists by itself being independent of us. By comparison, saññā-vedayita-nirodha has no such independence. When certain conditions are met, it arises. It is not self-existent. It is triggered by the efforts of the adept and possibly the particular posture and some other aspects.