Hi Chrysalis:
Good question. I've struggled with this myself. I still have a difficult time understanding sometimes how it should be used.
Lilly uses the increase (NN) and decrease (SN) interpretations. I am not overly fond of this particular interpretation myself as it doesn't seem to add much to the reading, imo.
I think I read in Olivia Barclay that the SN in the first house can indicate a mark on the querent wherever the sign suggests. So SN in Aquarius would indicate near the ankles. I've had this work for me personally at least twice.
I really like the Jyotish interpretation of the SN which is like an "empty promise" and this is the one that I most often use in horary interpretations, especially if it winds up in the house that is ruling the question. So SN in the 7th house with other slightly positive indicators may indicate that romance looks promising, but because of the SN nothing will come of it because the quesited cannot "make it happen" for some reason. I've had a chart where that happened to me personally too - the quesited's intentions were good but nothing happened.
When I see SN on the cusp of two that means to me that if the querent is expecting payment of some sort that it won't be forthcoming or may be less than expected.
I have also used the modern interpretation on occasion, specifically where Venus was translating light between two significators in a friendship chart. The nodal axis was in an exact square with Venus which I took to mean that "destiny" was against it and in fact it didn't happen. Later, in an online post somewhere I heard this called "the bendings" as in: "Venus is at the bendings" and it means the thing will be denied, but I have never heard or seen anything about it other than that one post.
As for the NN, I generally think of it as a lucky or beneficial place. In lost item charts it is considered a Fortune (a lucky indicator, like Jupiter or Venus).
Hope this helps and I hope some of this is accurate!
Cheers,
moonwitch