Learning relative pitch - the k-pop way

This is a music theory-related post.  I wrote it initially as an ask.fm answer to someone who asked about how to learn relative pitch.  I got halfway through and then thought to myself "fuck it - this information could be really useful so why not make a blog out of it so it's not buried 269 pages deep in my ask.fm answers list and impossible to dig out in a month's time.  People who are not musicians or aspiring to be musicians may struggle to understand this post and may also be incredibly fucking bored by it, and for that I apologise, but there probably isn't a way around this to be honest.