No I didn't say it was huge, I said it was big

Make me feel worse by ignoring that cheap title.

Anyway, I noticed the Overwhelming Critical feat and that it doesn’t stack with the Massive Criticals property, but I don’t see how the latter functions exactly. Does it just roll a 1d4 (in the case of Terrance’s Fell Axe) and add the result as extra damage, or what?