The first time during combat you are wounded Severely, is usually a breakpoint. You lose a lot of blood, probably, or might even have broken something. To avoid passing out on the spot, you must make an ability check on CON or WIL. Whichever is the best for you. You need to make 6 or better to keep fighting, otherwise you faint.

Note that this check is only for the first Severe wound in each fight. If you manage to stay on your feet the first time, you’ll take any beating that follows until you win, run away, or die.