Disclaimer. This is an analogy. Like all analogies it generalises.
Kata, for most of us, is fixed. It is a set construct that we learn and rehearse. It does not vary very much. Over time different instructors have figuratively taken the same block of ice and carved away at some of the edges, added on smaller blocks, broken it down into lots of blocks and reassembled it in a different way, or taken chipped off elements from lots of different blocks to form a new block for others to replicate. In this manner we have lots of stylistic variations on the same kata and new kata have been created. Because it has been frozen (fixed) and joined in different places at different times its crystals are generally not aligned and it is filled with air bubbles; the block is opaque.
Training regularly is said to polish technique. Training regularly in a kata does indeed polish the structure, it polishes the surface of the ice. You get to know the contours and positions, you can form them in your minds eye and they become ingrained. Polishing the ice has value for understanding the shape of the form. But form is not the same as function. Form is a dance that teaches important positions, movements and develops strength and balance – a combative dance but a dance nonetheless. Polishing the ice brings the satisfaction of the development of those attributes, it takes a lot of effort and brings clarity to the surface, but as with a lot of ice the interior remains opaque and hidden. The dancer cannot utilise the form outside of the choreography; to deal with the unpredictable they are forced to utilise other methods. Their kumite and/or self defence bears no resemblance to their kata.
As a state of matter, ice is limited. It is strong, incredibly strong, but not adaptable. It can be cut to fit shapes, but then is limited to those shapes. It is limited to predictable fixed scenarios.
There is a welcome increase in the interest in learning the applications of kata in karate at present. This interest itself is nothing new, but I would argue that for many years the explanations given to students were so ridiculous and ill-informed that they drove away from karate those of a practical and independently minded nature who were not prepared to overlook the deficit and simply continue to develop the attributes gained by polishing ice.
More than ever it is possible for karateka to easily find videos and books on karate application, and while there is exceptionally good stuff out there, it still isn’t all that common and it is often surrounded by the bad and the ugly. Even amongst the good, I see a lot of demonstrated applications produced by well meaning people that I regard as ice. They have simply chopped up the kata into smaller blocks and arranged each for static attacks. There is no evidence of adaptability, there is no provision for failure, a way of moving between applications is not taught. They have simply created more blocks of cloudy ice. It is simply a smaller dance routine. They have the shape of the form but cannot see through its substance.
To get inside the kata you have to do more than break it into blocks. You have to heat it up through training. You have to work those blocks through unpredictable and dynamic training until they completely break down and merge together into one transparent mass of water. Good application is like water. It moves freely, it fills and exploits spaces, and it continuously adapts. The tiny air bubbles and ill aligned crystals that made the ice opaque disappear, and the meaning and potential become clear. Applications should be fluid, they should be adaptable, and we should be able to flow like water from one to the next, nor be limited to one kata.
Once we have our water, our kata becomes something different. A medium through which we swim in our paired or multiple person training. We benefit from and utilise its substance, but it no longer constrains us with the rigidity of blocks of ice. Having heated it this way through our training, we can allow it to cool in a controlled manner into ice for our solo practice, and because we can control how slowly it cools and freezes in layers, we control its opacity. It is ice to polish once more in solo practice, but now it is transparent, and now we can see through it.
Kata may be ice. But be like water my friend.
John Titchen 
Hi John , yes I get all that and it makes sense . That’s why 3k has no engine as we put it,, Graham Palmer told me 3k has a lovely polished bodywork and paint job. But we build a v8 engine as well and put that into the body to make it work and run like a dream !! I like that description, it has stuck with me .
Hi Martin
I’m a yes / no on this. I know what you mean, I know what Graham means, I’m just not comfortable with trying to use 3K as a means of describing certain types of karate:)
Some people might describe the karate I teach as not 3K, and yet it is – I just interpret the K differently.
My Kata is the solo practice of the template techniques and principles.
My Kumite, literally grappling hands, is the paired application of that kata through analysis into effective and appropriate drills.
My kihon is the practice of individual techniques or combinations to improve their delivery – almost exclusively done through impact work against pads or people.
I tend to use the phrase Creche Karate for stuff that is all about appearance but very superficial in content or understanding – ie, if we are using Graham’s car analogy, the stuff that has no engine (and no suspension, and no grip).
All the best
John