When you click on that tool, little window pop ups (witch you can move like "PCB Tools" window and "Layers" window) where you can chose (typing in, not drop down list because it might glich and some net maybe won't appear in list) what net will lightup in "some color" and you can edit PCB while that window is open or closed to allow easy manual tracing. While auto router is running this tool is disabled.
Brand new interactions and interfaces. 
Easy to use and quick to get started. 
