I misspelled it. It's called butt wink. It happens at the moment I am breaking parallel. At this point my pelvic tucks under and the lower back loses its arc. I think it's normal to have a little butt wink, but mine looked really excessive. I am watching and reading some videos and articles right now for further info. Like this one http://bretcontreras.com/squat-biom...-is-it-what-causes-it-how-can-it-be-improved/ I will certainly consider that if I continue have problems. I'll try do it more slowly the next time. I hope so!! It's by far my favourite exercise! I hope I'll reach 60kgs again soon.