The idea being that you the basic idea would be on the normal tech tree line but there would be a branch of improvements in the tech that would be in the dead end line. For example chariots would be the basic tech as now but the "Celtic suspension" tech would be a dead end tech off it which allows chariots to perform well in more terrains. "Scything wheal blades", the favorite of films, could be another.
Sounds a bit like "Doctrines" from RI. I remember "flaming arrows" was one of them.