Editing mecanim animation from script

I am trying to edit humanoid animations from script. For legacy animations, it is no problem. But for mecanim animations, things get counter-intuitive. There are properties like Chest in-out, front-back, right-left etc. What do they mean? I am able to edit Transform based properties but how should I interpret something like in-out? Another way to solve my problem would be being able to create mecanim (retargetable) animation from scratch. I create an animation with same curves as a mecanim animation but it is still not retargetable. Is there anything else I should add? Any docs, links, examples are appreciated.