Each type of creature has its own weak spots -- places where the hide isn't as
thick, scales don't overlap, or where vital organs are closer to the skin.
Knowledge of these places is crucial to the archer who can't afford
diamond-tipped arrows for every shot. Steel arrows might shatter on the scales
of a large reptile, but will lodge in the beast's throat just fine.
Using this skill requires practice more than anything else. The best way to do
this is to find different types of creatures -- humanoids, mammals, reptiles,
etc -- and shoot them. It is easier to find weak points on slower, weaker
monsters, but you don't learn as much from them. Stronger, faster monsters
have better protected weak points, but you learn more by finding them.