"You're bound to become a buddha if you practice
If water drips long enough even rocks wear through
It's not true thick skulls can't be pierced
People just imagine their minds are hard."
- Shih-wu (1272-1352)
Allow me, if you will, to begin with an analogy. Let us say that you wish to become a doctor. How would you learn how to become one? I hope it would not be to watch television shows about doctors. It would probably be to study long and hard to achieve your goal. Now, during all of your study, would you strap on headphones playing loud disruptive music and read by strobe light? If you did, you wouldn't get very far.
This example is a direct parallel to trying to understand Zen without zazen. If you do not learn to quiet your mind, how can you expect to hear anything? Study without focus is merely mental masturbation. Why even waste your time, if you are not willing to put forth the effort?
I'll not get into the question of mind quiet, as I have babbled extensively on that idea in another node.
How do you perform effective zazen? Danan Henry Sensei once told me that to perform zazen at home is not enough for most people. They are easily distracted, and won't stay at it long enough. When I first heard this, I thought it was a mixed-truth, and one designed to generate revenue for the Lotus in the Flame Temple, of which he is the teacher. I have since discovered that he is right. In organized zazen, you must sit for the period of time allotted, unless you have an emergency of some sort. This forced situation lends itself quite easily to increased focus and progress. You are surrounded by fellow students, and many times, your sensei, so the urge to move is diminished. You are far less likely to scratch your itchy nose during zazen while participating with your peers.
Zazen is the foundation of Zen, and Zen is the foundation of zazen. You cannot seperate the two. Without Zen there would be no need for zazen, and without zazen, there would be none who understood Zen.