Zen meditation, as it is now commonly practised, does not work. It was asserted by a prominent Taoist meditator in Taiwan master Zhan RuoShui 湛若水, in his book Taoist Immortal breath (道家真氣).
Why? Before we can tackle this assertion, we have to answer two questions: How is Zen meditation (zazen) commonly practised today? What standard has been used when asserting Zen meditation does not work?
Nowadays Zen meditation classes make things easy for beginners. One sits down with or without leg folded, depending on the flexibility of each participant (participant as distinguished from student which implies more complex techniques needed to be learned). And a participant is asked to focus on his or her immediate/here and now, which when put into operational terms, usually means a focus on breathing and counting breathes if simple focus cannot drive away your disturbing thoughts. The following are typical instructions (I Googled from a Zen meditation site):
In zazen, we focus on the breath. Breath is the vital force; it’s the central activity of our bodies. Mind and breath are one reality: when your mind is agitated your breath is agitated; when you’re nervous you breathe quickly and shallowly; when your mind is at rest the breath is deep, easy, and effortless. It is important to center your attention in the hara. The hara is a place within the body, located two inches below the navel, inside the body. It’s the physical and spiritual center of the body. In zazen, you will begin to develop a relationship with the hara. You will practice putting your attention there; putting your mind there. As you develop your zazen, you’ll become more aware of the hara as the center of your attentiveness.
Does it work? Surely it does for some, assuming that enough practice time has been put into it. According to master Zhan, he knew many senior practitioners or monks of Zen Buddhism use this method. And they do get good chi generation and are able to get into a good trance state during meditation. Sometimes they call this direct method which is easy to learn. Hence one has only to participate in zazen rather than follow complicated instructions like the Taoists do.
In short, for the occasional participants, they can have a nice half hour's mental rest during zazen to refresh their mind, like going to a spa or soaking in a hot Japanese bath. And for the serious masters, they can get the same benefits (perhaps take a bit more time) as the Taoist meditators who (stupidly) trying hard to get chi going up and down.
So what is the problem?
Next we have to analyze the standard of Zen meditation. In all classic Zen and Taoist texts, the definitive guideline for good meditation is that one has keep the same internal sensation in all situations: Walking, living, sitting and lying down (行住坐卧).
The problem with modern zazen is that although a participant or a serious meditator can get into the meditative zone when his is doing zazen, when his mind is less than the almost sleeping state of zazen he will be out of zone. In order words, he cannot maintain his internal sensation when he is out of seated meditation.
In order that a person can be in the meditative zone when he is in a relaxed mood during walking, living, sitting and lying down, he has to train his breathing muscles to drive chi to the extremities of his body (in Taoist lingo, breathing with one's heels). It takes time and patient practice and will be impossible without the conscious stage of boosting up one's chi level. In Taoist lingo it is called Turning physical energy into chi energy (練精化氣), the beginning stage of the Taoist internal alchemical process.
To achieve this one needs much more training than just sit down, close your eyes, count your breath to drive away your thoughts. There are many ways of training. Master Zhan's suggestion in his book is to start with internal martial art before one tackles seated meditation. The easiest way is to start with zhan zhuang.