Five element theory is the daoist way of looking at life processes. The five elements are earth, water, metal, fire, wood.
(Note that one of the usual sequences for presenting these five elements is: fire, earth, metal, water, wood. This is known as the creative cycle.)
My own understanding of five element theory and how the elements fit together is based on trying to understand a set of basic principles that can be used in any context to make learning, doing, experiencing life, and understanding it more efficient, effective and enjoyable.
Look at this as an attempt at understanding, or putting things into some sort of useable framework to try to explain that understanding.
Lets say that you are learning a yoga pose or even a sequence of yoga poses.
The first thing is to have some sort of foundation (earth.) The foundation is more than just the base upon which the posture stands, whether you consider that base to be the earth or the bottom half of the pose itself.
Prior to laying a foundation, engineers need to know what they are building. They have to know the structure of what they are building. That can apply to doing a yoga pose. We have to know the basic shape of the pose that we are doing so that we can place our foundation appropriately.
As an example, in Warrior 1, if you know the basic shape of the pose, back knee straight, front knee bent, front thigh level with knee over the foot, spine and arms reaching upwards, you can position your legs appropriately and try to do the pose.
Because you know what you are trying to do, even if you only have a rough idea, you can work towards it. And you may encounter roadblocks, small problems that you have to deal with individually in order to progress any further.
In "engineering land" these are problems which designers might not have foreseen when drawing up the plans. Laying a foundation, who knows where or when you'll encounter sewer lines, cables, a subway tunnel or spaceship that no one documented.
That brings us to the next element, connection (water).
To build a pose you can focus on individual elements so that you can learn how to do them. This could be akin to a foundary producing I-beams offsite, or bricks for that matter, which are then taken to the building site to be assembled into a building.
In terms of a yoga pose,
You can practice these things individually so that you can focus on them, learn them and subsequently do them without having to think about how to do them. You connect to these ideas. Then you can put them together in a yoga pose like Warrior 1.
If you've ever learned to touch type, the idea is the same. Learn a few letters at a time, then you don't have to think about those letters in order to type them. They become built in. Your fingers do the work for you. Likewise, learning the elements of a pose. Learn them well enough and they do themselves.
Once you've learned the basic parts of a posture you can then unify those parts with an even clearer idea of what you are trying to do. Where a "plan" or "blueprint" can be considered a foundation of sorts, actually putting a building up is what unifies all the disperate parts.
Some people use five element theory to explain Tai Ji and a part of Tai Ji is understanding substantial and insubstantial. When you are substantial you have a center. All the parts of your body are connected and so can work together, unified by that center. Likewise, in learning a yoga pose, in taking a yoga pose from blue print to actuality, you can unify all of the separate parts and put them together.
One of the qualities of metal is that it pulls inwards. In the same way you can think of the idea of a pose pulling all of the separate parts together and holding them together. This is like the metal reinforcing rods that are used in concrete so that it can withstand tensile or pulling stresses. (The concrete itself can withstand pressure.)
With metal you can unify the parts of your body into the yoga pose that you are doing. You make it real.
Five element theory is often likened to the seasons. In spring and summer the weather slowly gets warmer. Plants expand outwards. In fall and winter the process reverses. Center could be considered the earth or the idea of cycle that repeats.
In which case earth should be considered center.
The beauty of five element theory is that it can be changed or adopted to suit a particular point of view. The important thing is to understand the point of view that you are using it from. So for this article I'll focus on assoicating metal with the idea of center or unity.
Other points of view can also be useful and also lend themselves to understanding. And that is ultimately what five element theory is about, a model for helping us to undersand.
While doing your pose you can work at creating space within it so that you can use your senses and respond to what you sense. You then have sensitivity and control. The idea of creating space can be represented by fire.
Ideally you can use the heat of a fire to create just the right amount of space. You have to finely control it. Not too much and not too little. This requires concentration and concentration is best fueled by passion. Make your posture big, but at the same time continue to feel your body and control it. Continue to connect to the individual parts.
With these elements in place you can express what you are doing. I chose to represent expression with Wood. You can express your self freely. Then you can rest and in so doing give space for others to express.
This is like a tree whose branches provide shade or support for the nests of birds. But also like a tree in that it moves outwards and then inwards, each expression each year providing the foundation for further growth in the years that follow. And then, even when it is dead it still has use whether as fuel for fire or for craftsmen or for someone to build a home out of.
In terms of yoga poses, expressing your body can mean expressing it depending on how your body is on any given day. Over the course of seasons and years, trees mold themselves according to the environment, according to what is happening now. A yoga pose can be done in the same way. If you feel your body now, you can base the way you do your pose based on what is happening now.
When learning a yoga pose, five element theory can be applied in such a way that foundation is the rough idea of what you are doing, the plan, connection is learning the separate parts, center is unifying those parts, space is giving yourself room to sense and control while expression is doing the pose itself based on what is happening now.
Five element theory can also be applied to doing. In this case foundation or earth can be considered the bottom half of the posture, the part that connects with the earth. Connection can be connecting and feeling each part. Center can be your center of gravity and feeling how it relates to your foundation. Fire can be creating space within your body so that the parts can move relative to each other. Expression can be the sum of all of these parts.