Um wait, do you mean "how do I know if pitivi is busy or if it's frozen" or do you mean "I want to know how the internals work"? In the first case you should file a bug report with debug logs (see the website in the "bugs?" section), in the second case you may want to ask around on the IRC channel with a more specific question... pitivi does not "run commands" to do the render, as I understand it.