A video that goes into insane depth into the cultural context in Korea that's behind this. This video is a part 2 and I do recommend the part 1 (which is a full hour shorter than part 2) as well. But part 2 is the one that deals specifically with this gesture.
It will probably not surprise anyone here to learn that it's a little more complicated than
a hand gesture used by a radical online feminist community almost a decade ago