Knockback
Knockback is the push a hit gives its target: a short slide away from the blow that shows how hard it hit and how heavy the target is.
Also called hit push, pushback.
When a hit lands, the target slides back a little. That slide tells the player the blow had force and which way it went. How far each body moves also tells them how heavy the target is, which matters as much as the size of its sprite.
How should knockback be split?
Treat a hit as two bodies trading momentum. Something lighter than the blow takes the whole shove; something heavier takes part of it, and the rest comes back at the attacker as a bounce. Jumping the target straight to its new spot looks like a glitch, so the push should be a short slide.
Knockback in Mytheria
A creature's mass comes from its size, its radius divided by the player's, squared, and the biggest creatures count as at least 14 times the player. Each weapon has its own strike mass: 3 for a sword, 6 for the Lantern Censer and 8 for the Stormglass Anchor. A sword hit throws a thornling 21 pixels while you don't move; the same hit moves the Stoneback Colossus less than 6 pixels and bounces you 9 pixels back. When the blade connects, most of what's left of your forward lunge is handed into the target's slide, so you stop as it starts to move.
The full post, with clips and demos How to make big enemies feel heavy in combat