setting TDiagramLine TextCellsMode

The source code for Diagram Studio has a number of properties related to accessing the TextCells of a Line such as SourceTextCell, TargetTextCell, CenterTextCell, etc. However according to the comments in the source code, these can only be accessed if the TextCellsMode of the line is set to tmSpecific. The problem is that I can't find a way to set the TextCellsMode property, it is not exposed in the TDiagramLine Type. There must be a way I am sure but it's not obvious to me. Can someone point me in the right direction? Or have I misunderstood the point/use of these properties?

At the moment I am using TextCells[0] to access the center text cell which is all that I need now but it would be nice to know for future reference....

TextCellsMode is targeted to help creating TDiagramLine descendants. You create a new class, and specify how you want your text cells mode in the inherited class.