Middling is when you take two different lines and hope the number hits in between. For example, over 193 and under 196 in an NBA basketball game and the score lands on 195. You can also play a side where you push one and hit the other. Example o195 and u196.

The mistake often made is playing the over 193 and then bet under the 196 when the line moves later in the day. Middles are only valuable when you play two lines in the same market and don't middle away an advantageous position.