Minecraft wiki entry for Things_not_to_do, associated text: Using a sword enchanted with Knockback on a skeleton knocks it away and gives it more time to attack the player, making it harder to kill the skeleton. There are exceptions, such as when the skeleton could be knocked back into a position which kills it or removes from where it can harm the player, such as over a cliff. They would likely die from the fall, and if the water broke their fall then they would not be capable of attacking the player (if the cliff was high enough). Also, a player with a shield could protect themselves against its arrow if the player was using a sword enchanted with Knockback.