Here are the official rules:
Method to Determine Big Ten Conference Automatic Representative to Bowl Championship Series - BIG TEN OFFICIAL ATHLETIC SITE
The BCS consists of five (5) bowl games: BCS National Championship Game, Rose Bowl, Fiesta Bowl, Orange Bowl, and Sugar Bowl. Participation by a Big Ten Conference member institution will be determined as follows:
BCS National Championship Game. In the event the conference has one or two football teams ranked No. 1 and/or No. 2 in the final BCS poll, these conference team(s) shall participate in the BCS National Championship Game.
Rose Bowl. Unless ranked No. 1 or No. 2 in the final BCS poll, the conference champion shall participate in the Rose Bowl. The championship shall be determined on the percentage basis of conference games (tie games counts ½ win and ½ loss). If there is a tie for the championship, the Rose Bowl representative will be determined as follows:
An ineligible team shall not be considered in the standings for determination of the conference representative.
If there is a tie for the championship, the winner of the game between these two teams shall represent the conference.
If there is still a tie, or if the tied teams did not play each other, the representative shall be determined on the percentage basis of all games played.
If there is still a tie, the highest-ranked team in the final BCS standings shall be the representative.
If more than two teams tie for the championship, the same selection procedures shall be followed with the following exceptions:
If three teams are tied, and if one team defeated both of the other teams, then that team shall be the representative.
If three teams are still tied, and if two of the three teams defeated the third team, the third team is eliminated, and the remaining two teams shall revert to the two-team tie procedure.
If three teams are still tied, and there is a tie game between two of the three teams, or if two or all three of the teams did not play each other, the representative shall be determined on a percentage basis of all games played.
If three teams are still tied, and one of the three teams is eliminated through the percentage basis of all games played, the remaining two teams shall revert to the two-team tie procedure.
If three teams are still tied, and all three teams have the same winning percentage of all games played, the highest-ranked team in the final BCS standings shall be the representative.
If four or more teams tie for the championship, the following selection procedure shall be followed:
If one team defeated each of the other three teams, then that team shall be the representative.
If two of the four teams defeated each of the other two teams, the latter two teams shall be eliminated, and the two remaining teams shall revert to the two-team tie procedure.
If three of the four teams defeated the fourth team, the fourth team is eliminated, and the remaining three teams shall revert to the three-team tie procedure.
If there is a tie game between two of the four teams, or if two of the four teams did not play each other, the representative shall be determined on a percentage basis of all games played.
If one of the four teams is eliminated through the percentage basis of all games played, the remaining three teams shall revert to the three-team tie procedure.
If all four teams are still tied and have the same winning percentage of all games played, the highest-ranked team in the final BCS standings shall be the representative.