The idea of snaplines to me is access the simple line at the center of the timber. I place plumb and level lines on the ends of the timber. On an 8x8 timber for example the lines will be 4" from the reference faces on each end, on a 10x10 they would be 5".... The lines on each end will be level to each other.
Then the lines represent extensions of the simple line at the center of the timber.
To lay out a mortise or a tenon, I would use the snapline as the inside of the mortise/tenon on an exterior frame, or the center of the mortise/tenon on an interior frame.
Here is a sketchup model that uses snaplines, with centered m&t, as in an interior frame.