A good bookmaker will set up odds so they appear enticing to both sides of a bet: if Arsenal plays Stoke in the Premier League, the bookies want an equal amount of action on both clubs, the better to spread risk. (This isn’t how they make money, however. They do that by designing the odds so each bet results in a small percentage going to the house, called the vig or the juice.) If more money than expected goes in on Stoke as game time approaches, bookies will adjust the odds to make Stoke less attractive and Arsenal more so, ensuring things remain balanced.
—Brian Blickenstaff writing for Vice about match fixing in the sports world.