In Pathfinder, the Trample ability in the universal monster rules states that it works "just like" the Overrun combat maneuver. However, Overrun is a combat maneuver pitting CMB versus CMD as usual, whereas Trample is an ability that can be resisted with a Reflex save. With Overrun, the target is knocked prone if the attack exceeds its CMD by 5 or more. What happens in the case of Trample, since there is no combat maneuver roll at all? Is the target knocked prone if it fails its Reflex save? As a GM, that's how I would rule it, but strictly by RAW it doesn't seem to be the case.
Borrowing from this question, below I've modified the steps it presents to jibe with how I think the trample process works. Be aware that the combat maneuver overrun is plagued with inconsistencies and lacks clarity, so the GM must address some parts of it because the rules don't. Nonetheless, there is some designer input on this particular issue: in a 2010 thread Paizo creative director James Jacobs says that trample victims "aren't automatically knocked prone."
Steps in the Trample Process
The creature possessing the extraordinary ability trample takes a full-round action to move—likely only up to its speed, but ask the GM—, its path occupied by at least one foe of at least one size category littler than the creature.
The creature enters a square the foe occupies, provoking an attack of opportunity from the foe. The GM may rule that a creature possessing the feat Improved Overrun doesn't provoke an attack of opportunity from the foe.
- A foe that makes this attack of opportunity suffers a –4 penalty on the attack roll and is dealt the creature's slam damage + 1½× the creature's Strength modifier.
- A foe that doesn't (or can't) make this attack of opportunity makes a Reflex saving throw (DC 10 + ½ the creature's Hit Dice + the creature's Strength modifier). Failure means that the foe is dealt the creature's slam damage + 1½× the creature's Strength modifier. Success means that the creature's dealt half damage.
The creature continues its movement, restarting the process upon entering a square of a foe that this round has not suffered the creature's trample damage.
A creature can enter squares occupied by the same foe multiple times during a round (likely provoking an attack of opportunity only the first time, but ask the GM). However, a creature can deal its trample damage to a foe only once per round this way.
What This Means
Because there is no combat maneuver check made, there's no chance for the creature to fail an overrun attempt (so, for example, the creature can't be returned to its starting square) nor for the creature to succeed on the overrun attempt by at least 5 (which would render the foe prone—this renders the feat Greater Overrun largely pointless for the creature, too).
Trample has the same effect as an overrun combat maneuver (including knocking enemies prone), with added benefits, but only works against smaller creatures than the user.
From the Universal Monster Rules :
As a full-round action, a creature with the trample ability can attempt to overrun any creature that is at least one size category Smaller than itself. This works just like the overrun combat maneuver, but the trampling creature does not need to make a check, it merely has to move over opponents in its path. Targets of a trample take an amount of damage equal to the trampling creature’s slam damage + 1-1/2 times its Str modifier. Targets of a trample can make an attack of opportunity, but at a –4 penalty. If targets forgo an attack of opportunity, they can attempt to avoid the trampling creature and receive a Reflex save to take half damage. The save DC against a creature’s trample attack is 10 + 1/2 the creature’s HD + the creature’s Str modifier (the exact DC is given in the creature’s descriptive text). A trampling creature can only deal trampling damage to each target once per round, no matter how many times its movement takes it over a target creature.
Format: trample (2d6+9, DC 20); Location: Special Attacks.