I am not terribly sure what it entails. It seems to be an Australian competition, possibly pony club related? I have not seen any advertised in the states, although I haven't coached pony club in about five years.
I know huntseat and stockseat jackpots are where you pay a certain entry fee, higher than for other classes, and the winner gets 40% of the total, 2nd gets 20%, 3rd gets 10%, etc. Perhaps this is similar?