Piker Circle Packing

I'm just playing with circle packing, and I found this gif by Daniel Piker on this thread: http://www.grasshopper3d.com/photo/circle-packing-1

Could some body please explain to me the basic logic to this gif,  i understand the triangulation and circle creation, but what creates the expansion and stopping at the boundary curve?

I just need some steering or even component name so i know what to research.

Thanks