시선 one’s eyes / attention 잊을 만하면 If someone is about to forget/ almost forget 한번씩 once 뒤통수 back of a head 따라붙다 to chase close / follow meaning overall, the eyes followed the back of the head when it was almost forgotten. you almost forgot but it got back to you. I need the whole context to .. give you the more detailed nuance. If you have more questions, we can also talk about it in the lesson^^ see you in the less! xD