The overkill system, is when you fight a monster that will give you exp but do more damage than its max HP.
When you do more damage, you get the extra dmg that you did as an exp boost. Here is an example.
In this image, I do 120 damage to the unfortunate Blood Jelly, and gain 324 exp.
But in this image, I do 206 damage, and gain 503 exp, from the same monster as before, at the same level.
Using this system, you can milk some more exp out of the monsters you fight 
