That means that your development be it art or science, social or physical depends on the various activities you perform, if you practice math and architectural design, you will definitely understand more theoretical structures and systems.
So when you practice programming or design, it isn't that you increase your Ti or Te, it means that you think and use creativity or rules to applied problem solving, which then later may be reflected in how you do other things too.
Let's say I learn programming skills (I take a programming course). that definitely increases my skills in creating software. So I guess we can say my ti on creating a software got developed, because now I can analyze, categorize, evaluate better and faster according to the new principles which I learned on the course (-which happens to be the definition of ti function).
What has any of this to do with strengthening my ti function in a general context? I strengthened my ti function relating to software, because I can create them more easily and faster. But when faced with problems that have nothing to do with programming my ti hasn't changed. I cannot provide better insights on physics problems after taking the programming course.
the reason for this happening is that what I have learned on the course is just knowledge. this kind of knowledge is incompatible with physics and that's why I don't become better at solving physics problems after talking the course.
So, after the course I have obtained new knowledge. I have also obtained better concentration skills. I don't think I have developed better analyzing, categorizing, evaluating skills (- the definition of ti function) apart of in my field. I think that the latter skills (ti) has been my necessary skill set to understand programming when entering the course. I believe my ti helped me obtain my new knowledge more easily, since it is easier to understand programming language if you have ti as your primary function.
Some people think that ti (any function) should operate as a muscle. In this case that means that using ti because I am learning programming strengthens it, just like an athlete strengthens his muscles when running. I don't get why functions should act like muscles? there is a biological explanation why muscles get stronger after using them. Does this biological explanation apply to functions as well. I think not, because functions have to do with psychology, not biology. So why do people keep thinking that using your function will strengthen it? As I said, you use your ti to learn new knowledge and the results you get are: new knowledge, concentration skills. By what logic is your function strengthened as a byproduct in this process?
Apart from new knowledge I agree that you grow some new neurons. those can help you in analyzing, categorizing, evaluating (-ti function). So you can say, "you see, you increased your ti by taking the course after all". But wouldn't that mean that creating new neurons= strengthening ti? Does that make sense? Could you equalize a function to something like your number of neurons? If not(which I expect you to agree), by what logic is your function strengthened when you use it? It's not a neuron and neither a muscle after all.