If one thinks of the consecutive nature of Si in its relations, it is easier to see the difference in ISTp and ISFp. ISFp tend to revel in what feels good from instance to instance -- it is like hedonism in tiny, tiny fractions between then and now. ISFp's tend to take note of these patterns elsewhere and try to recreate it in the near future via the choices they make now. ISTp's on the other hand, are more likely to recount the truths between instance to instance after thinking about it after some time.

This is what I have noted when they are in a relaxed mode -- in other words, this is what they prefer they wish they were doing if they were not going full force at, for example, work.