If a character takes lethal damage equal or greater than half of their CON score, rounded up, they must make a DC 10 Strength check. Upon failure, the character will fall prone. They only make this check if they are standing or crouching when being hit. If they are crouching they have a +2 bonus to the check.
If a character takes lethal damage equal or greater than their CON score, they are knocked prone and make a Fortitude save equal to the damage dealt. On success they are dazed for one round, on failure they are stunned for one round.