http://en.radiovaticana.va/articolo.asp?c=668214
and from the article
Returning to the Motu Proprio, it has been established that once Conclave has started a two-third majority votes is required for a valid election, “calculated on the basis of the electors present and voting" and that if after three days the votes do not come to a result, "one day should be dedicated to prayer, reflection and dialogue". In subsequent rounds of voting, "only the two names which in the previous election had the highest number of votes will be considered, and the provision of a two third majority of the Cardinals present and voting for a valid election will remain”. Finally, the Motu proprio states that the two candidates who receive the most votes cannot vote in the final run-off election.