Forcing a backseat landing causes it, but can be impossible to prevent sometimes. (for example, backslapping a monster drop and NEEDING to ride it out, because there's trees in the landing that you don't feel like tomahawking into.)
I made a thread on it here:
If you don't feel like reading it, I basically say this chinese analgesic medicine numbs your shins for temporary relief if you need to compete, or want to ski injured. (not recommended, but if it's a powder day, vacation, or competition, It's worth the added recovery time from ignoring the pain and shredding anyway.)