In a betting exchange you can bet against a horse, usually in betting and in horse racing people need to guess which horse would win the race - so here you bet on the horse that you think won't win the race.
So I'm going against horses with very heavy odds on them (e.g. 100 to 1 or so).
So I'm going against horses with very heavy odds on them (e.g. 100 to 1 or so).