You can make a bull rush as a standard action or as part of a charge. When you make a bull rush, you attempt to push an opponent straight back instead of damaging that foe. You can bull rush an opponent who is up to one size category larger than you, but not one larger than that.
INITIATING A BULL RUSH
You move into the defender’s space, provoking an attack of opportunity from that defender. You also provoke attacks of opportunity as normal for your movement. Attacks of opportunity made by anyone other than the defender against you during a bull rush have a 25% chance of accidentally targeting the defender instead, and attacks of opportunity by anyone other than you against the defender likewise have a 25% chance of accidentally targeting you. When someone makes an attack of opportunity, roll to see whether the attack went astray, then make the attack roll against the appropriate AC.
RESOLVING A BULL RUSH
You and the defender make opposed Strength checks. A combatant receives a +4 bonus per size category it is larger than Medium or a –4 penalty per size category it is smaller than Medium. You get a +2 bonus if you’re charging. The defender gets a +4 bonus if it has more than two legs or is otherwise exceptionally stable.
BULL RUSH RESULTS
If you beat the defender’s Strength check result, you push the defender back 5 feet. If you wish to move with the defender, you can push it back an additional 5 feet for every 5 points by which your check result exceeds its result. You can’t, however, exceed your normal movement limit.
The defender provokes attacks of opportunity if it is moved. So do you if you move with it. The two of you don’t provoke attacks of opportunity from each other for this movement.
If you fail to beat the defender’s Strength check result, you move 5 feet straight back to where you were before you moved into its space. If that space is occupied, you fall prone in that space.