I - Rules
(see JITD, p. 22; FAQ, p. 2, 6)
After inflicting at least 1 damage (before applying the effects of armor) to a figure with a Knockback attack, the attacker may immediately move each affected target figure up to three spaces away from its current location. The figures must be moved to spaces that do not contain other figures or obstacles that block movement. The figure does not actually move through the first two spaces - it is knocked completely over them. As such, this "knockback movement" is not blocked by any intervening figures or obstacles (though a figure cannot be moved through a closed door or wall).
Knockback and Large Monsters
Large monsters (monsters that occupy more than one space) are heavier and therefore less affected by Knockback. For each space beyond the first that the monster occupies, it is moved one less space by Knockback. For example, an ogre occupies four spaces, so it would move three fewer spaces when affected by the Knockback ability. A dragon, on the other hand, occupies six spaces, so it would move five fewer spaces.
Can an attacker using Knockback move the target in any direction, or just straight back? Also, can the target be knocked back into a pit?
The attacker with Knockback can move the target in any direction, including into a pit or other trap.
II - References
Heroes with the Knockback ability: none.
Monsters with the Knockback ability: Ogres.
Skills granting the Knockback ability: Water Pact (attack with Staff only).
Items granting the Knockback ability: Hammer, Mace of Aver.