I view that as a set of actual graces, and often the ordinary workings of Providence.  So, for instance, God determines the Pope that will be elected, arranges circuмstances so that he won't err (advisors he talks to, experiences he has, etc.), and, worst case, God can have him die first if he's dead set on defining some error.

Yeah, I'm not sure I've ever read a detailed explanation of papal infallibility in terms of anything that happens to the pope ontologically.  It's not like the authors of Sacred Scripture who were inspired.  Popes are simply prevented from teaching error.  How?  I don't know, but I don't view it as some inherent quality of a Pope as Pope, but rather more something that God protects the office from.