-pinterest removes pages that contain the term pinterest, so pages from a pinterest.* domain, plus articles about pinterest, etc.
Sounds pretty good to me. If I'm searching for a term, but don't want results from pintrest, then I'm probably not wanting to read an article about pintrest either.
"Here's a picture I took today. I also posted it on Instagram and Pintrest."
If you wanted to read an article about Pinterest, it would probably be hard to search for it in any other way.
Except that the word "pinterest", like pinterest.* results, appears in all the nooks and crannies of the rest of the Web. Excluding the domain like that succeeds in narrowing the scope of the search without excluding content unduly.
I use that a lot. Wouldn't Google notice if a lot of people were telling it they don't want to see results from Pinterest and make some adjustments to their SERPs?