The people I know that are Si dominant seem to have the following characteristics.
They make their decisions based on what has worked for them in the past, and they know what works. Once they know how to do something, they can be incredibly efficient in repeating it. They like the familiar vs. things that are novel or that they have no experience with. They can be resistant to a new way of doing something or an approach they view as unproven. In general, they seem to have an enormous store house of knowledge or recollection of past experiences that they can rapidly recall and apply to current situations. It can make them real experts.
I find them to have very definite views, which appear to be based on their past experiences as to how something should be done. STJs can be very directive in this regard with others - there is a way something should be done (more task oriented) and they make sure others do it that way. SFJs seem to have a different approach to things - not so much task directed. Their approach seems to be that people should behave in a certain way and they make others aware when they are behaving in ways that are not aligned with this accepted way or cultural norm. In general, I find SJs to be a bit on the stubborn side.
In my view, the way the function works in supporting decisions is that it compares the current situation to stored impressions of similar situations in the past. That comparing and contrasting of the past to the present is the essence of the function. The people I know who have this as a dominant function tend to have very strong impressions or memories of situations. It is a curious thing that sometimes, these memories are flawed, but it is clear that these previous experiences, however they recall them, are what shapes how they make decisions. What also is curious that it can very difficult for them to accept an alternate recollection of past events. It is as if they are absolutely certain their memory is correct (after all, who has a perfect memory!?).