Units with Trample inflict 5 + 2 * size of trampler untyped AP damage when moving through another unit. As with usual damage calculation, both sides get a DRN added to their roll. However, unlike other damage, trampled victims always take at least 1 damage, regardless of the roll results. Trample ignores shields.
Then there is a check of the trampled unit's defence skill - (1 per 10 fatigue) vs 3d6 (exploding) to take only 1 damage1). The trampler wins ties. Weapon bonuses and penalties to defence skill are ignored, but armour is respected.
Trampling units only use their other attacks when meeting a square containing at least one unit of the same size or larger as themselves.
Trampling units can trample several squares per turn, the maximum amount is dependent on their combat speed. Specifically, the delay between successive tramples (in ticks, of which there are 7500 per round) is calculated as follows2):
The final delay is calculated as:
Combat speed delay + Action rate value/size penalty
As the size penalty is always at least 3, it means that large amounts of combat speed alone will not greatly increase the number of tramples per round.
Every square trampled costs full encumbrance in fatigue.
Trampling units can not be repelled but still have to test against awe. Effects that trigger when a unit is damaged (such as Blood Vengeance) work normally, but those that specifically trigger on melee attacks such as Fire Shield do not.