Above the blogpost editor I see such information:

Use on an empty line to denote the end of the short version of this article

[cut]

I have 2 questions:

1. What is it used for? I suppose this 'short version' of the article is the one displayed when the article is searched or similar?
2. When I try to use it, either text before it becomes very big or nothing happens (see above). Is this the intended behaviour?

Thanks!