This is actually quite useful for me on a 'type finding' standpoint. It explains a lot that the introverted perceiving functions are more 'hardwired' into the core psyche than other types.
Its a new insight that helps me narrow down type choices (And that studying functions on other sites isn't as good as reading posts about the same topic here )
From what I can tell, I use Si when I right and spell. Its common for people to ask themselves 'does this look right?' when they spell something, I think this is Si.
I can't remember things without some sort of reaction agent, like something in a conversation might suddenly spark a memory of something that happened soooo long ago. But I could never remember it consciously.
While typing: I just suddenly remembered playing a videogame when I was real little and living in a different house, all I really 'see' is the game and my hands - this led to a memory of my first bout of bad dehydration. I'll cut this off before I get mental diarrhea and dump all over this post.
Si can also be something that makes a person 'comfortable' with something. Like a friend always seems to call randomly during the day, at first it's annoying but now its almost routine. When they stop you feel there's a void 'what happened?'