Potion of Knockback is a new potion made from Slime blocks.
If you drink it, you'll get sent flying 16 blocks in the air.
The splash potion is way more useful,
When you throw it at your opponent, he gets sent flying in the opposite direction. It has 2-3 times the knockback power as the Knockback II enchantment has. You can use this on yourself to have similiar effect as the regular potion.
Witches are a bit too weak once you get close to them. They just keep drinking their health potions and won't attack as long as you hit them with a sword.
So I think Witches should be equipped with Potions of Knockback, and they should throw them at the player, if he gets too close to them.