We have no difficulty understanding that the author of a book can express something, or that music can express something especially with the lyrics, or painting can express through symbols and images. The difficulty begins when we think about abstract arts, like abstract painting for example. Abstract painting usually is just blots of colour, sahpes and lines, without representing an object of the world, and therefore without any meaning that you could immediatley read from it. Often the painter him/herself can´t tell you what it means. And just so, Parkour is just shapes in space and time, without any obvious meaning.
And yet, they are both expressions, IF the final picture is the result of a inner process of selecting by preferance. The artist adds and erases over and over again, until s*he feels that image is now finished. An adequate represenation of his inside, so to say (some artists might already have a finished image in mind, before starting to paint...rather rare I guess).
So the final picture is not simply randomness (as it would appear to the observer, especially the ones that have no connection to art), but it always stands in relation to some part of the artists subjective experience. And yet, we wonder, what information such an abstract image could possibly carry?
First of all, I´d like to quickly explain abstractness. Something is abstract, if it has no objective representation. For example harmony is an abstract concept, because there is no object which is harmony itself. Objects can show forth harmony, but there is no harmony just standing around like a chair. Just as concepts like justice, beauty or happiness are not objects. Tree and stone and house, on the other side, are words relating to actual objects in the outer visible world. And so they are not abstract.
Some people like to think of abstractness as being something foreign and far from practical daily expererience, but the opposite is true. Abstractness is the closest thing to us, given that all feelings are abstract, and even experience itself is something completely abstract. You could say, that we are swimming in abstractness. It is our natural element.
Words words words
Syllables, by the way, are themselves just abstract symbols! We just coded them with some meaning. Sometimes we get tricked into feeling that if we have a word for something, it is more of a something than things that don´t have words. And therefore when we think about expression, we probably think about our spoken and written language first. Because it is the most obvious and familiar tool that we use to "share one´s mind". We can simply say, what is going on in us, what ideas or feelings we have.
But the fact is, humans experience more, than is expressable with words. You could say, experience has a higher resolution than our spoken and written language – and therefore words are often very inaccurate and ineffective in expressing all that makes up the subjective human experience.
Example: Happiness is pretty intangible, if you think about it. Yet, it is very concrete in our experience! Happiness has a very specific "taste" you could say. And that taste of experience we have coded with the syllables of "h a p p y". But there are many shades of happiness that taste differently that all go under the heading of "happiness". So the word "happiness" is just a very crude simplification of the actual experience of happiness.
My theory: There seems to be an inherent urge in humans to express their subjective experience. And as a result of that, different languages have evolved that express different things that make up humans inside experience. So, different languages are designed to carry different subjective content so to say.
Maths for example is the result of people being engaged in thinking about abstract ideas like logic, law, order, relations and what not. Over time this subjective experience of mankind has lead to a system of symbols, known as maths. Maths is a language that fulfills the necessary requirements that a language needs, to be capable to express ideas of a mathematical nature. And it will evolve further, to be capable to encompass new ideas.
Music is another non-spoken language, and it seems to express a different part of humans subjective experience than maths (even though they are somehow connected rumours say). It can carry emotions much better than maths (though I´m not a mathematician… I might be wrong).
Languages enrich each other, for example mathematical concepts also brought a lot of new vocabulary to spoken language (like addition, subtraction, fraction, potency, degrees, principle, etc). So languages refine each other. Also they can cross-over, so that the possibilities and precision of expression is often increased. For example written language, without pronounciation and body language can often be ambiguous, cause it is missing a great part. You know how cold casual text messages without emojis can sound :D
Other common cross-overs are for example dancing, film, music with lyrics.
I think people feel attracted to those languages, that are expressing those things, that correlate with what they have on the inside. It is obvious again, if you look at music. You can love Hip Hop, but absolutely not resonate with heavy metal, even though both are music. But they are expressing different vibes and you might just not feel the one vibe. You have not connection to it, you can´t feel it, you don´t have it inside of you. And so it is, that Parkour people are somewhat movement lovers, but dancing can leave them completely cold. Yet, some Parkour people that are prone to dancing, might lean towards dancy Parkour. Can you see how the chosing of a language is already an act of indirect expression?
(for a more detailed explanation of expression, check out: Parkour, sport or art).
Ok, ok ok, you want words from me. Actually I wanted to finish the topic by the rather brutal statement:
Parkour is expressing that which makes you feel like doing Parkour.
But probably you will blame me for playing with words and not saying anything :D (But you would not be so suspicious when I would say, feeling like dancing makes you dance).
So I will try to pinpoint some feelings, that we do have words for.
Parkour has many forms with different aspects, so you can conclude that different people have different feelings that are expressed through Parkour.
Some are all about aesthetics, composing, sort of sculpturing beautiful lines into the environment of spots. I´d call it an aesthetic feeling. Feeling of harmony, beauty, order. Certainly related to those feelings that lead to other aesthetical art forms like music, sculpturing, architecture, painting, dancing... That alone is a pretty big chunk already.
Some Parkour people are more about the complexity of challenges on a mechanical level. Parkour to them is like solving riddles, lock-picking, crossword puzzles :D So, whatever feeling is behind that drive, it might be related to the feeling that makes mathematicians have joy while solving a mathematics problem.
Then there is of course also the emotional part to Parkour. Some Parkour styles, you can see easily, are coloured by emotions. It can be feelings of elegance (yes, it is a feeling) or more martial animalistic feelings. Mystical feelings, coolness and basically any feeling that you can have while moving. Yes, you can even feel Hip Hop while moving. I wonder what word could be used for how Polianski feels when he is in his movement character.
Then, another big one: flow. Flow is something that is feelable, too. Just like musicians can feel rhythm and melodies in their inner ear, advanced flowers feel rhythms and melodies in their... inner body? :D By the way, Mandalas, or any of these ordered patterns that mankind has created, all point to the feeling of order. And flow, could just as well be seen as ordered movement.
And then you can even feel twisty or linear, swingy or jumpy, playful or straight-forward. Yes, you can even feel like impact. I have that sometimes. :D Then you can feel creative. The creativity mood is one that makes you want to play around and do unorthodox things. A feeling that makes humans do all kind of weird stuff that leads to all sort of crazy discouveries :D
Also Parkour can be crossed with other channels of expression than movement. For example with videography. If the mover is also about the picture the movement creates in the camera, he will not only think about the movement, but also in what way a line is filmed, what angle, and also at what spot, to create his picture. It is sort of a cross-over between movement and painting.
So, Parkour can be an expression on many levels. The complete picture of the movement of a mover is the the outcome of a great mix of things, a multidimensional image. Some people might have more dimensions than others, depending on how many aspects of their inside are influencing their movement. For example, there are very unemotional movers, and very emotional movers. It doesn´t mean that the unemotional mover has no emotions :D, but rather, that he doesn´t take his emotions into consideration when moving. Or it is simply not the factor which makes him move. His/her interest is rather caught by another aspect, that is expressable through Parkour.