An example
One book has the Eagles -2.5. Another has the Cowboys +3.5. Bet both. If the Eagles win by exactly 3, both bets win.
What it costs
Most of the time the score lands outside the gap. Then one bet wins and one loses, and you lose about the vig on the pair. A middle is worth it when the chance of landing in the gap beats that cost.
Key numbers matter
In football, games end on 3 and 7 far more than other margins. A middle that covers 3 or 7 is worth much more than one covering 5. Our board only posts middles where the gap and the prices make sense.