Aloha Nacho,
I had to grab my "little sister" who works next to me at LSC to show her the position and ask about the back.
The tendencies are to be on your toes and learning forward.
You want to keep the feet flat and work down until your more vertical. Your feet should stay parallel rather than be pointed out. This will take time to work into that position, as your opening up that same sacrum area. Be gentile.
I could put another animated gif for it along side the bouncing. 
(I'll see if I can find a visual refernce on the web)
Much Love, Shawn
