Pope Francis is shaking things up in the Catholic Church to such an extent that many talk about a “Francis revolution.” Yet the single most revolutionary act committed by any pope in at least the last 600 years fell exactly one year ago tomorrow, and it wasn’t Francis who did it.

On Feb. 11, 2013, Pope Benedict XVI used a meeting of cardinals discussing new saints to deliver the stunning announcement that he planned to resign, effective 8 p.m. Rome time on Feb. 28. The news was a total surprise to everyone except a handful of papal intimates, and it set the stage for all the drama that’s followed.

One cardinal said afterward that he sat in the room well after the meeting broke up, still unable to comprehend what had just happened. He played Benedict’s Latin phrasing over and over again in his mind to be sure he’d understood.

Yes, a handful of popes had resigned before, most recently Gregory XII in 1415. The circumstances, however, were so wildly different as to make Benedict’s decision essentially unprecedented – a pope not facing foreign armies or internal schism who decided voluntarily to step aside, while continuing to live on Vatican grounds and pledging “unconditional obedience” to whoever might succeed him.

Francis wins plaudits for his humble nature, but Benedict’s act was arguably the zenith of papal humility. He’s gone from infallibility to near-invisibility, having been photographed just four times since the resignation, most recently at a Jan. 15 musical recital marking his brother’s 90th birthday.

In the immediate wake of the announcement, the game was afoot to identify the “real” reason Benedict quit. While the pontiff cited age and health, some observers wondered if he was so demoralized by the surreal Vatican leaks affair — which ended in the arrest of his own butler as the mole — that he couldn’t go on. Others speculated it was a nebulous “gay lobby” in the Vatican that had brought him down.

Most of that ferment circulated in the Italian press, where no conspiracy theory is ever too wild to get a hearing.

Whatever the reasons, we can see more clearly today how Benedict’s abdication prepared the way for the choice of Cardinal Jorge Mario Bergoglio of Argentina as the new pope.

First, making resignation a live possibility effectively took age and health off the table as voting issues. In the past, conventional wisdom had been that the cardinals would prefer a candidate in his mid-to-late 60s. Older, and they run the risk of a short papacy or one immediately submerged into a health crisis; younger, and the danger is being stuck with one leader for too long.

Resignation provides an exit strategy, either way. If an older candidate gets sick, he can step aside and end the paralysis that comes with a weakened pope. A younger candidate can resign after the creative arc of his papacy is over, making way for a new direction.

Without such a release valve, the cardinals might have hesitated before electing a 76-year-old missing part of one lung, especially after choosing a 78-year-old, in Benedict, who occasionally seemed to lack the energy to get the Vatican under control.

Second, the fact that the cardinals were electing a pope after resignation rather than death changed the psychology of the process.

There was no outpouring of grief and tributes to the deceased pontiff, for the obvious reason that the pope wasn’t dead. There were no vast crowds of mourners in Rome, no appreciative obituaries in the global press, no emotional crescendo of a funeral Mass – none of the forces that can make it more difficult for cardinals to opt for a break with the papacy that has just ended. Eight years ago, the massive “funeral effect” surrounding the death of Pope John Paul II was a hugely important factor in shaping a continuity vote.

Resignation allowed the cardinals to take a more critical view, which helps explain why the 2013 conclave was the most anti-establishment papal election of the last 100 years. In this case the cardinals weren’t rejecting the teaching of Benedict XVI, which most of them admired, but patterns of business management in the Vatican they believed had become corrupt and dysfunctional.

They wanted change and Francis is delivering, perhaps to greater degree than some of them anticipated.

Finally, resignation encouraged the cardinals to roll the dice on a Latin American outsider with no Vatican experience because, frankly, some had in the back of their minds that if the new pope turned out to be a flop, they could come back a few years down the line and pick somebody else.

Catholics traditionally believe the Holy Spirit guides the process of picking a pope. On a more worldly level, however, the prime mover in the chain of events that led to Francis actually was Benedict XVI, the improbable revolutionary, who set the wheels in motion one year ago.