I’m entering – I agree it’s a good spur to hone pitch, first chapter etc. Picking 500 words is always tricky… I think the judging of that ends up so subjective!
In terms of the 150 word pitch, yes, think about it as the blurb on the back of book. A tried and tested structure for this is: set-up and protagonist; inciting incident; will she / won’t she hook. For example:
Since the death of her father, poor Cinderella’s life has been nothing but drudgery. Treated like a slave by her ugly step sisters and cruel step mother, all she wants is freedom and love. When the Prince of the Realm announces a lavish ball, Cinderella is sure she will lose out once again, until her fairy godmother casts a beautiful spell, gifting her the means to attend. At the ball, all her dreams come true: she dances with the Prince who falls head over heels for her. But the fairy’s gift only lasts until midnight… can love prevail once the spell wears off?