I believe you hav to use roller blades constantly to get access. In Lumiose City, there's a girl with roller blades in a yellow outfit where you hav to learn a move called back flip from her. After that you gotta learn another move called spin or wtever in Laverre City. I'm not sure if my answer is correct but when you passed the punk what you get in there is a roller blade move called cosmic backflip hence I assume its related to roller blade skills.
