Source: Unapproachable East
You know how to launch a devastating attack from above by dropping onto your opponent.
Benefit: You can execute a charge by simply dropping from a height of at least 5 feet above your opponent. For example, a ledge 10 feet above the floor of a cavern would suffice for jumping on a Medium-sized creature, while a ledge 15 feet high is required for a jumping on a Large creature. You can't jump from more than 30 feet above your opponent, nor can you effectively battle jump while under the influence of a fly or levitate spell or effect, as you have to hurl yourself down on your foe.
If you hit, you can choose either to deal double damage with a melee weapon or natural attack or to attempt a trip attack. You are treated as one size category larger than normal if you try to trip your opponent with the battle jump. After you attack, you take falling damage as normal for the distance you jumped. You are entitled to a Jump check (DC 15) to take less damage, as if you had fallen 10 feet less than you actually did. If you fail this Jump check, you fall prone 5 feet from your opponent.
You can also use Battle Jump to begin a grapple attempt instead of making a normal attack. If you do, you are treated as one size category larger than normal for the first grapple check following the battle jump.
Normal: Anybody can try to jump down on an enemy, but it is not considered a charge, and they do not gain double damage or the size bonus for the ensuing attack.