A derby is like a small cross country course with both solid and knockable jumps in an arena ii normally has a water jump to. It requires great control as some jumps are very flimsy it also is usually timed and based on overall look so you have to look nice and in control overall clearing jumps easily and clean. Your horse should look like it can jump a hunter track cleanly in an arena space.
It will also have a showing session after to ensure your horse is a correct mover and you can control him. THere is a confirmation check as well.