Can someone describe to me what cheapshot does and how it is most useful?

Given the extremely high amounts of thief points, one would assume it is quite powerful. But the description is basically that you kick people while they're down...

Like when? When tripped? When else? I don't see thieves knocking people down all the time.

Is there a lag affect? Anything else I should know about what it will and won't do, what builds are best for it, before I spent all the points on it?

Thanks. I still have a lot to learn.