I haven't found too many personal descriptions of what this function is like if it is first position
Here are examples where I sense it working:
I consider SI to be the RAM(Random access memory) of a computer - and this leads to:
1.) Immaculate memory - I have memories as young as two years old - and many from childhood. I'm very good at things like Jeopardy, Scrabble, crosswords - although I find them boring and don't like watching or playing these things. I can remember facts that I learned years ago.
2.) I get sensory "rushes" for things that greatly excite me. For instance, if I come across unique architecture in real life, I will get a surge of energy through my whole body, where I can sense the architecture through my eyes, ears etc.. Same things for books or other objects, or art. These sensory rushes can be negative. When I get them, time stops, and I become loss with no thought or feeling for a period of a few seconds to minutes.
3.) Can immediately sense what type of mood someone is in and it can wear off on me.
4.) Very health conscious - If I eat bad, or processed food - I can feel the negative effects on my body immediately. If you can sense what fast food, soda etc you stop eating that stuff easily.
5.) Can reproduce different states in my body at will. Excitement, anger, passion, etc.
4.) If I'm in a bad mood, I can bring everyone down with me. Don't need words - just the visage on my face does it. Same goes for good moods.
5. People constantly complain that they can't tell if I'm listening or just ignoring someone. I frequently am in my head but still hear and notice everything. They are always surprised when I bring up something they said but they thought I wasn't listening.
6. Can distinguish quality vs bad quality in anything
7. Remember that typing doesn't speak to the QUALITY of the functions, just preference. Although I prefer, SI, my EN is very strong. Get it out of your head that people are either sensing or intuitive - we ALL use both.