there is a hierarchy of "knowing". we use words for concious thought. but words, whether spoken out loud by ourselves or others, or silently to ourselves, are symbols, not things themselves. they are at best an approximation or a concept of real things. and concious thought only takes us so far. if we had to use our concious thought process to do everything, we wouldn't be able to do even simple things, like walking or eating. there wouldn't be enough words, and we couldn't think them fast enough. our bodies rely on unconcious thought to function. the transfer from words or concepts to concious thought and finally to unconcious "thought" is the process we use to learn to do things. when that process is initially completed, we say that we can "do" something. we can then conciously tweak the process. to do this, we have to go back to the realm of concious thought and concepts to modify what the body does without thought. "more air"..."less pressure"..."eee to ascend" etc. only when these new concepts become re-assimilated by the body through repetition, have we really learned whatever refinement we were trying to accomplish. this process is continual, every time we attempt to do something. every repetition is crucial, because our body is continually "learning".
so to the original question, why we only seem to know something when we can do it: when we are introduced to the idea of doing something by using words and concious effort, our first attempts are likely to be somewhat crude. the words are inadequate, and concious thought is too cumbersome. only when the task is assimilated into the body through many repetitions do we have any consistent success at doing. at this point we may describe the process with the same words, or we can find better ones, but the process we are actually using at that point is completely different than when we first started. that's why somebody who can do something may describe it in simple words, and those words may or may not actually mean anything to us. alan vizutti may say, "range is just faster air." upon hearing this, can we play like alan vizutti? no, not so much. his words mean something specific to him, not so much to you. they are just words, at least at the beginning.
this is only a third of the story at most, because while we are doing something we can simultaneously evaluate what we are doing. this evaluating process goes through the same stages as doing. at first we evaluate simply, thinking in words, like "the note i am playing is flat." at some point we learn to take corrective action. we conciously lip the note up. eventually, this also becomes automatic. even when we do something as simple as playing duets, we learn to constantly evaluate. are we in tune? are we with the other part rhythmically? shap, flat, high, low, swinging, not. we eventually drop this inner dialogue, simply listen and make these corrections without concious thought.
and while we are doing and evaluating, we also have the capicty to respond emotionally to what is happening in real time. frustration or nervousness produces tension in the body, for instance. we learn to manage our emotions. once again, we start with words or commands developing into an inner dialogue. eventually we assimilate our inner dialogue so that it becomes automatic and hopefully beneficial, rather than an obstacle.
this is one hell of a question, and i am doing it no justice with my awkward words here. but the discussion about what it means to think, and the nature of the dialogue between the mind and the body, have been around at least since aristotle, and continues with books like the one rowuk refers to above. it is something that as musicians, we constantly explore and try to use in our favor.