Over Hit

If you succeed a roll with 10 or more over the mark, double the damage. As an example, if the AC of a target is 15 and you roll a total of 25 or higher, your damage dice are doubled.

This also applies to spells that do not have an attack roll but require a saving throw. An example is if the the spell DC is 15 and the the opponent rolls a total of 5 or less, double the damage dice.

This feat may also apply to Critical success or failures and is known as Over Crit when applied.

As such if you roll a natural 20 but your total for your dice roll also exceeds the mark by 10 or more, all of your damage dice are doubled for that attack including the critical damage dice.

This again applies to spells without an attack roll but require a DC saving throw. If the character rolls a nat 1 but their modifiers would bring their total roll to be less than 10 points from the DC, then Over Crit does not apply. However if the critical 1 and modifiers is 10 or more points from the DC, Over Crit applies.