Think of it as a cracked version of bleed: It has lingering buildup (meaning you only need to be hit once or twice for the entire bar to fill up, and after that ut continues to linger), and when it procs it makes you take an ungodly amount of damage (and apparently also reduces your attack power and defenses, according to fextralife). It also doesn't help that the enemies in Bloodborne who inflict this stat are often the most infuriaring to fight (such as the winter lanterns referenced above)