Middling is a betting strategy that exploits a shifting point spread line, or a moving totals line. A middle occurs when you place two opposing bets on the same event. In this sense, middling is a hedging strategy, but the goal isn't reducing your exposure.
A potentially very profitable situation can occur in sure betting and they are called middles. A middle is an arbitrage bet where if a match ends with a certain result, you can either win one bet and get the other bet refunded, or win both your bets. Read all about middles in the Ultimate guide to sure betting.