It's based on it, but doesn't require as much to pull off. That one requires you to reduce a creature from full hit points to 0 in a single hit, then if the attack roll was high enough, you can hit another creature within reach that was adjascent to the previous and deal the excess damage to it. If this one also dies, you continue the chain until the original attack roll stops hitting or you fail to drop a creature from full HP to 0. Because this is potentially easier to do, it spends a feat and has another feat as a prerequisite.