-ㄹ락 never travels alone: it pairs with 말락 to describe something hovering right at the edge of happening. 비 올락 말락 해서 그런가? 'maybe it's because it's about to rain but not quite.' Every line here is that doubled shape, describing a state caught halfway — nearly asleep (들락말락), a toenail nearly lifting off, a face nearly coming into view.