Kathrin, I found this video helpful in identifying the cause of my butt wink –

http://www.mobilitywod.com/2013/09/pro-episode-42-rounded-lumbar-in-the-squat-hows-your-hip-flexion/

Coupled with this one for techniques to improve hip flexion –

http://www.mobilitywod.com/2011/06/episode-262-hip-flexion-case-study-and-mobrx-part-2/

I also found that box squatting to a depth where I don’t butt wink, is helping to reinforce good technique while I work on my mobility.

Here’s a good box squat vid –

http://www.mobilitywod.com/2013/09/pro-episode-46-the-box-squat-knee-function-rehab-pain-and-pathology/

Cheers