Trip

Trip has a chance to trip the target, rendering it prone for a longer period of time than Trip. It has a cooldown of 15 seconds. This feat is granted for free to Barbarians, Fighters, Paladins, Rangers and Rogues.

In game description: Using this attack, you may trip the target, rendering it prone. Some creatures may be immune to this effect.

Game Mechanic is as follows:
 * A attack roll is made against the target. Like any melee special attack, it works with your combat flow and is queued to work on your next melee attack, ideally you will time this so that it will work at your maximum attack bonus.
 * If it hits, it will do normal damage, and in addition attempt a to Trip the target.
 * The target then attempts a strength or dexterity save (whichever is higher) against your DC - this is now displayed in game, DC for trip is 10 + Strength + Related enhancements + Vertigo weapon. Should the target fail its save, the target is tripped.
 * It will remain tripped for approximately 12 seconds, during which time, the target is rendered prone and may not move or attack. Prone targets suffer -4 AC. Attacks/spells already started will complete, however.
 * Prone creatures are given an additional balance save to get back up every 2 seconds - the DC on it is the same as the trip DC, but it uses the balance skill rather then Strenth or Dexterity.

Official: Some creatures are immune to the effect - Mostly this means red and purple named creatures are always immune. But some other monsters that have no legs are also, such as ooze.

As of module 5, size doesn't directly effect your chance to trip, mostly this means Halflings do not suffer -4 to there trip attempts for being small. However larger monsters, or monsters with many legs are given appropriate bonuses to resist the trip, +4 per size difference or +8 or greater for creatures with many legs.