talchaさん explained very well with the words and details.. If I'd put them in a sentence, I'd.. うしろを振り返ると、 When I looked back, 少女が胸を張り断髪を肩に振り乱しながら走っている。 a girl was running with her chest sticking up and her short hair was jumping in the air (above the shoulders).

『うしろを振り返ると、 少女が胸を張り断髪を肩に振り乱しながら走っている。』 =>> I look back when a girl shaking rumple hair has been running. She is chest out posture while running. Her hair style(called 断髪〔danpatsu〕) is short, cut straight around the neck. うしろ　…　one's back 振り返る　…　look back / turn around 少女　…　girl 胸を張る　…　chest(breast) out posture 断髪 …　In this case, 断髪 is a kind of hairstyle. 　└→　http://images.google.com/images?hr=en&q=%E6%96%AD%E9%AB%AA%E3%80%80%E6%98%AD%E5%92%8C&um=1&ie=UTF-8&sa=N&tab=wi 振る　…　shake 乱す　…　rumple # I couldn't explain by the natural sentence of English since my English is poor. Can you feel some of flavor of it?