The original Lecture here.
Startpoints of our diagonals. = All the points except the whole last branch and all the last poits in all the other branches.
Endpoints of our diagonals. = All the points except the whole first branch and all the first poits in all the other branches.
Here are some more useful tutorials: http://atlv.org/education/grasshopper/
Download:
06 - Diagrid Data-tree