(Sebastian:) Above all, my congratulations to the nice page! My concern is: Since, as you write, this is a fundamental concept, I find it hard to believe that there isn't anything on SL describing it. Did you check this?
unkx80: There are several pages describing the concept, unfortunately, the information is scattered around in different pages that are often not linked together. Hence this page is a good attempt at bringing information together.
unkx80: I fixed the example in the original page. In the original diagram, White can play to . If at , then at .