Also known as a “Polish Middle”, an inverted middle is very risky to bet on. If the “middle” in an inverted middle occurs, you could lose both your bets, hence causing severe damage to your bankroll.
An inverted middle occurs when the ‘-’ side is bigger than the ‘+’ side (for handicap bets). For Over/Under, an inverted middle occurs when the Over is bigger than the Under (for instance Over 3.5 and Under 2.5).
If you flip the handicaps in the middles example, you get an inverted middle.
Do not bet on inverted middles if you are not 100% sure of what you´re doing.