Middling requires you to bet on both sides of the same bet after the line has moved, meaning that both wagers could pay out. For example, let’s say the Philadelphia Eagles open as the 2.5-point favorites against the New York Giants, and you bet on them to cover.
The line then moves, and the Eagles become the 4.5-point favorites. You can then bet on New York +4.5, meaning both bets will pay out if the Eagles win by either 3  or 4 points.