In WPA that governs our Official OP rules, the position of the balls are based upon 'the base of the ball where it contacts the cloth'.
The head string in considered outside of the kitchen. The head string is a line through the center of the dot or diamond.
Therefore when having Ball in Hand behind the Line:
1. An OB can be legally first contacted if its base is upon the line or below the line. 'Below' is towards the foot end of table.
2. The CB has to place behind the line, or 'above' the line, therefore its base can not be on the line, but needs to be slightly behind the line.
The WPA Ball in Hand behind the Line is so out of line of how Americans traditionally play BIH behind the Line, I hate to even comment on it. We have discussed this thoroughly a few times. Whitey