I'd suspect that there's a listing somewhere that says, for a given cell, what other cells depend on it, so it knows to update the others when it's modified. I can't immediately get it to work on my Mac, so I can't test it out, though.

There is a line in the source (_datastructures.py) that generates a KeyError("Circular dependency at %s" % str(key)), so I'd guess that it won't let you do anything that generate cycles on the dependency graph. And once the graph is acyclic, it should be fairly obvious in what order to update things.