From the official website
During the 3-hour finale nationwide simulacast on May 23 at 8PM EST/5PM PST, there will be one last vote to determine who will be crowned the next American Idol. Voting will stay open throughout the show and close during the finale's last commerical break. However, one of the Top 3 will be eliminated part way through the show, based on the results up to that point. So be sure to get your votes in early. If your favorite goes homes, don't worry; you can still vote for the Top 2 contestants.
I do wonder whether this favours Grace and Willie because if one of them gets eliminated the other one left will get their votes.