The Matter of Zen
Trying to explain Zen in words is an oxymoron. It is like the taste of chocolate. To those who have never tasted it no amount of words can explain it. To those who have tasted it none are necessary. Still we have to say something...
Though phenomena is constantly changing, the universe itself does not. I can burn a pile of leaves and obviously the leaves change but the universe is unaffected; nothing has been added or taken away. In the modern age when we are concious of our carbon footprint this may seem obvious. Thousands of years ago it was a radical insight. This lead to the formulation of the Two Truths Doctrine in Buddhism, in which on one level (the phenomenal) enlightenment and everyday life forms a kind of duality but on a deeper level (the noumenal) enlightenment and everyday life is one.
Traditionally Zen work meant koan work. Koans are paradoxial stories or questions turned over and over in the mind until some insight tumbles out. Every answer is subjected to the sword of discrimination until one is found that can withstand it. What is Original Purity? Who am I? Does a dog have the Buddha Nature? Do I have two kidneys? Do my two kidneys have me? I am not the body. I am not the mind. I want a donut, I eat a donut... Every answer leads to more questions.
There is a reason why this kind of practice in Japan is called 'Jariki' or Self Power and part of the Path of Arduous Practice. It can be exhasting and can only work in close relationship with a teacher who also had genuine insight. However there is another form of Zen practice called shikantaza which grew in popularity even as Pure Land teachers such as Honen and Shinran began to teach the nembutsu as Tariki or Other Power which is the Path of Easy Practice.
Much like Quiet Sitting, Shikantaza means 'Just Sitting' and involves watching the mind. If the goal of Zen is the direct insight into the nonorigination of all things (IE the noumenal) and the universe is in essence nondual as Buddhism teaches, then it is sufficient to be just be still and observe the mind itself.
In Japan the different Buddhist schools tend to remain distinct but in China and oddly enough more importantly in Korea, dual practice of Pure Land-Zen is common. I will get back to Korea in a moment but the Chinese Pure Land Master Han Shan wrote "We should, therefore, practice both Buddha Recitation and Zen. This is a proper and safe approach. One who can practice Buddha Recitation and then observe where Amitabha Buddha comes from and where He goes will, over a period of time, come to understand what Amitabha Buddha represents. This will allow his innate, bright wisdom to flow forth." 
There is a break in the continuity of Buddhism in China. In 845 CE there was a populist movement within China that attempted to drive out what was seen as foreign religions and monks and nuns were forced to return to lay life. Fortunately the persecution was shortlived. Unfortunately it was not the only such incident there. However Buddhism had long since became part of Korean culture and when we see dual Pure Land-Zen practice in Korea it becomes clear that this is not a hodge podge of ideas born of the confusion resulting from the persecutions of the Tang Dynasty.
In fact one of the vows of Amida Buddha is that all bodhisattvas who hear his name will attain direct insight in the nonorogination of all Dharmas. Honen in this regard wrote to one of his followers:
Nembutsu is the practice taught in the essential vow of Amida Buddha. Other religious practices such as observing the precepts, reciting a sutra, chanting dharani, and meditation on the noumenal aspects of reality, are not prescribed in the essential vow. For this reason, one who aspires for birth in the Pure Land must recite the nembutsu first. It is acceptable for one to further perform other practices in addition to the nembutsu, if one wishes. 
 Joji Atone and Yoko Hayashi (translators), The Promise of Amida p. 386