This is one it took me quite a while to figure out.
So, figure you have an ISTJ. In fact, I'll use my brother as an example. He sits at the same desk every day, for hours and hours. He always places his cell phone 4 inches to the right of his mousepad. He refuses to get a new mousepad, even though he's more than once spilled milk on, which has since soured, and smells. As an experiment, I made a small mark, in sharpie, in the corner of his LCD.
How Si got him to where he is. Si saw every miniscule detail in the desk -- he refused to use any desk that we hadn't already had in the house. He'd already absorbed a lot about the desk itself. Every time he passed by, sat down, or just looked at the desk, he collected, and stored away plenty of sensory information. This collection works a lot like a collage. It's the impression of an ideal like all introverted functions. In this case, the sensory ideal. He'd become used to the desk, and every other desk was inferior, because it was missing several of the components that he had assimilated into his understanding of how the ideal desk should look.
His mousepad, which he refuses to change, moved with the desk, and will likely move with him when he leaves the house, along with the desk and his computer. The reason is, he's compiled a collage of sorts. He's used to the mousepad, and slowly acclimated himself to the sour smell of the milk. He actually likes it, because for him, in his subjective interpretation of how the ideal mousepad should look on the ideal desk was built on this slowly developed collage.
Imagine taking a snapshot of a bridge. Then coming back a few days later and doing the same. Maybe there are different cars on the bridge. Maybe there's a biker or a few pedestrians. Perhaps the paint has started to fade. Si will notice all these things, and compromise the old with the new as long as it doesn't violate the old too strongly. The more often Si film is exposed to the light of the same object, the more strongly developed the collage is. It captures a new detail -- whether it's happened recently or was always there -- every time. It matches this up against the old blurred ideal, and eventually incorporates it.
Now there's more to it than that.
Si does store the collage of what is, but the information that's actually sent to judgement is not that. The information that judgement receives -- and this is why we say "it's like noticing when something is different" is that exactly. Si takes the collage and adapts it to the form of a model which can be used by Te and Fe. It tells either of these functions what's different, so they can go to set it back to what they're acclimated to.
It's why most businessmen insist on handshakes and suits and such. All a bunch of S_Js, and it's what they're acclimated to. Nevermind if it's effective or productive. Te's directive is led by perception, just like with an ENTJ. But the ESTJs perception is reestablishing what he's used to, or occasionally expanding, and ultimately doing the same in the new endeavor. What he's spent every minute of his life getting used to. Te is what forces those things to change back to their ideal, or attacks those things that won't.
This is why Keirsey says S_Js like to acquire things. They'll never let anything go, because they've become so used to it. As far as they're concerned, the wall behind that dresser doesn't exist. The dresser is part of the room. They might put something on top of it, sure, but it will be something miniscule like a dish or a trophy. Something that doesn't really violate the sensory ideal; that they can quickly get used to.