Normally every time you take damage you get a short invincibility frame (the grace period) in which you cannot take any more damage. However, this only applies if the incoming damage is lower than or equal to the previously taken damage, otherwise you take the full damage from the incoming shot.
This can be quite frustrating as it makes being shot more random, you could take little damage given the enemy with the high damage shoots you first (so low damage follow-ups will be ignored) or very high damage if you're shot in ascending damage order.
This mod changes that so that you only take the difference in damage if you would take damage during an i-frame:
In vanilla if you took 50 damage and then took 70 damage during the i-frame you would suffer from 120 total damage despite being in an i-frame. With this mod you would only take the additional 20 for a total of 70 damage.
In exchange for taking less damage during the grace period, the grace period will no longer be extended when being hit with additional damage during it.