Dirty Bit Propagation

I have been looking for more information on how and when Maya marks nodes dirty. First thing I have come to understand is that for an attribute two dirty bits can be set, connection and datablock. See isDirty command for these two options. I am still not clear how using these two flags Maya decides to call compute method. And what has confused me more is that if I don't mark a datablock clean then the compute method does not get called after first compute call.
Here is an interesting discussion about dirty propagation,
http://forums.cgsociety.org/showthread.php?t=119449