You earn from Polish Middles as with normal middles, when both (sometimes more) opposite outcomes occur.
Example of a normal middle: There are two bets: Total Over 0,5, odds 2,05, and Total Under 1,5, odds 2,50.
If exactly one goal is scored in this game, the result will fall under both outcomes, and thus the player receives the winnings twice. That is, it falls into the middle.
In the case of a Polish Middle, the calculation scheme is somewhat more complicated (it is described in detail on the PolishMiddles page).