This help topic is Open Game Content, and is licensed for public use under the terms of the Open Gaming License v1.0a .
Make the melee touch attack roll for a trip attack. The Str check is also made, although if the attack has failed you may disregard it.
Defend a trip attack. Makes a Str or Dex check (whichever has the higher modifier), modified for size. If this beats the attackers Str check, the trip fails.
A character can try to trip an opponent as a melee attack. A character can only trip an opponent who is one size category larger than a character, the same size, or smaller.
Making a Trip Attack: Make a melee attack as a melee touch attack. If the attack succeeds, make a Strength check opposed by the defender's Dexterity or Strength check (whichever ability score has the higher modifier). A character gets a +4 bonus for every size category he is larger than Medium-size or a -4 penalty for every size category he is smaller. The defender gets a +4 stability bonus on his check if he has more than two legs or is otherwise more stable than a normal humanoid. If a character win, a character trip the defender. If a character lose, the defender may immediately react and make a Strength check opposed by a character's Dexterity or Strength check to try to trip a character.
A tripped character is prone. Standing up from prone is a move-equivalent action. A character may make a trip attack against a mounted opponent. The defender may use his Ride skill in place of his Dexterity or Strength check. If a character succeed, a character pull the rider from his mount.
This attack form substitutes for a melee attack, not an action. As a melee attack, it can be used once in an attack or charge action, one or more times in a full attack action, or even as an attack of opportunity.