It lasts for only 2 mins 22 seconds. It's really hard to say anything that makes sense - because Fumiko no Kokuhaku definitely is not about sense, deep feelings and all that.
Story is as simple as a hamster - it's cute, yet very sudden. It starts with a girl's confession to a boy she likes. He says no - and she starts to run. And run. And even more run. And soon she just can't stop making her - very fast - way down from a hill. The truth is - all you get is just about a minute and a half of crazy run.
Animation is very cute. I personally loved it a lot. It fits the story and characters.
I guess, best way to give it a description is to say 'If you watched FLCL, imagine a non-scifi, very short and very simple version of it'.