This skill is used by biomancers to assess what percentage of hit points a monster has left. It is an automatic skill. Whenever you use the scan command, it will check your skill. If it succeeds, it will give you an indication of the monster's condition.
Fairly handy, who doesn't want to see how badly they're beating up a monster? You can use the look command to get a rough estimate, but this will get it down to the closest 10%. Doesn't actually help when you know you can take an area, but is useful to see if you're making any headway on a boss, or if it's going to take forever.