There is no explanation for the answer, which doesn’t explain much, so for those wondering, this is the SO page with an example of how rows can be moved to columns using that technique: https://dba.stackexchange.com/a/164794 .
Is it still not very obvious, but at least some info on that.
Course: https://www.educative.io/collection/5352985413550080/6335716377231360
Lesson: https://www.educative.io/collection/page/5352985413550080/6335716377231360/5255114844536832