From DDO wiki
Using this attack, you may trip the target, rendering it prone.
- An 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 only a regular damage hit (will not produce offhand damage or glancing blows), and in addition attempt to trip the target.
- The target then takes a Strength or Dexterity save (whichever is higher) against a DC of 10 + Strength modifier + related Enhancements + related item bonuses. Should the target fail its check, the target is tripped.
- The target remains tripped for up to 30 seconds, during which time it 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 a Balance skill check every 2 seconds to stand up early. The DC on it is the same as the trip DC, but it uses the Balance skill rather then Strength or Dexterity.
- Some creatures are immune to tripping, such as those that have no legs (like oozes) and those that naturally hover or levitate (such as mephits and ghosts). Red and purple named creatures are also immune.
- Your character's size doesn't affect your chance to trip. Mostly this means Halflings do not suffer -4 to their 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.
- Has a greatly reduced duration when used against players (~3-8 seconds).