The advice is to write straight from the heart and not to sit there pondering. Thoughts won't count unless you convert them into words. The story can be any length you like. This is usually pretty short because of the limited time, but some fast typists manage an impressive amount. Brevity, however, doesn't count against you.
The focus I feel during that time is intense and exhilarating. I am almost always pleased with the result. The exercise is brilliant for ensuring you craft a great beginning and a perfect ending. These seem even more important than usual in such a short story. You don't have long to make your mark.
Of course the middle is vital too. There has to be a progression for the main character and some sort of dilemma to sort out, just as in any short story. The MC has to move forward towards a credible and satisfying conclusion. Quite a tall order for a tiny tale.
I love 5.30pm on Saturdays. I love the challenge and the thrill of the writing, never sure what kind of story it will be. I have written crime, comedy, tragedy, romance, domestic and surreal. I never know what will be on my screen at 6pm.
The submissions are read and whittled down to a shortlist of three by the following Wednesday at 5pm. The un-shortlisted entrants then vote for their favourite. The winner, who receives a cash prize, is announced on Saturdays just before the new competition begins.
I have won a few times and been runner-up on a few occasions as well. It's quite an honour, because lots of talented writers take part and the standard is very high. I keep all my stories and often expand them to submit elsewhere. They are like miniature works of art when they turn out well.
It's a fantastic brain exercise, because you have to think so quickly and there is no opportunity for procrastination. The site is WriteInvite.com and I highly recommend it.