This is a structure that is a pole structure (or post structure). Considered by many to be the strongest and most stable. Also, for wooded areas, the quickest and cheapest. The major expense is really the roof. But that gets addressed in PSP.
"Shore" has to do with the style of walls (and, it turns out, the roof). Basically, good ole wood. So you end up with a structure that looks a bit like a log/wood cabin on the inside. And then you are going to put the weight of lots and lots of soil/dirt on it. But soil touching wood causes problems so you use ....
Polyethylene: aka black plastic. There is clear plastic too. But it is amazing how if you are familiar with the bits and bobs of this stuff, how it can be a powerful tool.
The first book goes into a lot of detail about why a conventional house needs a foundation - and all of the issues that crop up and all the things one does to mitigate all of those issues. It gets pretty complicated and you need to know what you are doing or you are going to screw everything up. Then it compares PSP and shows how a lot of those problems just don't exist.
I think calling these structures "underground" is a misnomer. Sure, you can use these techniques to do an underground structure ... and it is wise to not create too tall of a structure. But you can create a structure with light coming in from all four directions. The key is to use a bit of brains. With that bit of brains, you can build a house with one third the effort and one quarter of the materials. Plus the house can be made with 80% to 99% of materials right off your land (assuming a forested property).
The key is the roof. A green roof with 18 inches of soil and some polyethylene. The downhill path of the soil must always go to soil - not to any man made barrier. As long as you adhere to this, you can have a freaky cheap and easy green roof.