This help topic is Open Game Content, and is licensed for public use under the terms of the Open Gaming License v1.0a .
Make a bull rush: a Strength check modified for size and charging (+2). A character can bull rush as an attack action or a charge action.
When a character bull rushes, the character attempts to push an opponent straight back instead of attacking him. A character can only bull rush an opponent who is one size category larger than the character, the same size, or smaller.
Initiating a Bull Rush: First, the character moves into the defender's space. Moving in this way provokes an attack of opportunity from each foe that threatens the character, probably including the defender. Any attack of opportunity made by anyone other than the defender against the character during a bull rush has a 25% chance of accidentally targeting the defender instead, and any attack of opportunity by anyone other than the character against the defender likewise has a 25% chance of accidentally targeting the character.
Second, the character and the defender make opposed Strength checks. The character adds a +4 bonus for each size category that the character is above Medium-size or a -4 penalty for each size category that the character is below Medium-size. The character gets a +2 charge bonus if the Bull Rush is attempted at the end of a Charge. The defender gets a +4 stability bonus if he has more than two legs or is otherwise exceptionally stable.
Bull Rush Results: If the character beat the defender, the character pushes the defender back 5 feet. If the character wishes to move with the defender, the character can push the defender back up to a distance of an additional 1 foot for each point by which the character exceeded the defender's check result. A character can't, however, exceed his or her normal movement limit.
If the character fails to beat the defender's Strength check, the character moves 5 feet straight back. If that space is occupied, the character falls prone in that space.