I'm am not up on this event.
Simple break down:
You vote for your top choices number them, effectively, top choice, second choice, third choice. If no one gets over 50% of teh vote, the person w/ the lowest "first choice" votes is dropped and their second choice is counted. This happens until someone gets over 50% of the vote. That person is declared the winner.
With all the problems we have with voting already - apparently it's just too complicated - moving to this system is likely going to be a completely CF.