I am sure the line on the mat stack must mean something but I have no Idea what it is? I asked DD and she didn't know. Are they supposed to be over the line, not over the line and what is the deduction if they aren't?
I have always thought the line was what they have to land with their hands on the other side of. Not completely sure, though. As far as deductions go, I really don't know. I was talking to a coach once about the level 4 vault and commenting that it seems hard to tell if a vault is good or not, and she told me all the different deductions that can arise, and I was very surprised, especially on level 4! Who would have known - it seems like a cut and dry skill!
It is a deduction if your hands go past the line. Obviously any other part of your body is ok, since you land on the other side. I believe the deduction is about the same as going out of the floor area, but I'm not sure. I just know there is a deduction and tell my girls they don't want to go beyond it.