The Greyhound Bookie System
Take a race with 6 runners. Put runners No 1, 3 and 5 in one group, and runners 2, 4, 6 - in another group. Note which group has lower aggregate odds, then Dutch-back this group.
Apparently the idea is to back on the three selections which have higher chances of winning the race.
And here is an example of how the bets might look.