Damage an enemy by running over it with whatever steed you happen to be riding. A fast, cheap skill, but not spectacular damage wise, especially if riding a sheep.
The in game help reads:
Skill duration: 2. Type of skill : offensive skill. Party rows : 1st. Affecting stats: Dex. It uses 10 endurance points. The mere mass of a rider and his mount can be enough to crush his opponents underfoot (or hoof, as the case may be). A skilled rider can use his steed's weight as a weapon, either with a running start or by rearing up. Either way, the effect can be devastating. Of course, knowledge of combat riding is required for effective use of this skill.