As you might know, regular middles are positive arbs with a middle. For instance, you could bet on an American Football middle:
Tennessee (-7) @ 1.91
Pittsburgh (7.5) @ 2.14
If the middle doesn’t hit, you will make your regular arb percentage (0.92% in the example above). If the middle does hit (Tennessee wins with exactly 7 points), you will win your bet on Pittsburgh and get your bet on Tennessee refunded (making a profit of around 50%).
A negative middle has a negative arb percentage, but the spread can be bigger:
Tennessee (-6) @ 1.91
Pittsburgh (7.5) @ 2.04
This middle has a negative arb percentage (-1.36%), but the middle is 1.5 points. In this example, if Tennessee wins with 6 points, you will make a profit of around 50%. But if they win with 7 points, you will win both your bets (making a profit of nearly 100%).
Think of negative middles as a long term investment
In the second example above, the negative middle was -1.36%. This means that if the middle does not hit, you will lose 1.36% of your total stake. This may sound pretty bad, but think about it for a second; You only need to hit the spread once every 73.5 times in order for the arb to be profitable in the long run.
An additional upside on negative middles
As negative middles aren’t obviously in an arb situation, they usually stick around longer than ordinary middles and arbs. This means more time for you to bet on them, and less risk that the bookmakers will get a whiff of what your up to!