CLEAVE [GENERAL]
Prerequisites: Str 13, Power Attack.
Benefit: If you deal a creature enough damage to make it drop (typically by dropping it to below 0 hit points or killing it), you get an immediate, extra melee attack against another creature within reach. You cannot take a 5-foot step before making this extra attack. The extra attack is with the same weapon and at the same bonus as the attack that dropped the previous creature. You can use this ability once per round.
Special: A fighter may select Cleave as one of his fighter bonus feats.
So if you had an enhancement on your weapon, or a similar feat, to 'drop' the enemy you would be able to get a cleave attempt. What abilities can be used in tandem with this feat? Would a knockback weapon [what book is that from?] or a clouting [sp?] do the trick?
Dropping an enemy seems to be undefined, could knocking them away give you the same bonuses? Because then you are free to use the momentum of your weapon to hit someone else [which really seems to be the spirit of the rules behind cleave]