English Earnings at Polish middles, as well as at ordinary middles, occur because the event's outcome "overlaps" in both (sometimes more) differently directed outcomes.
An example of an ordinary middle: there are two bets: Ts 0,5, odds 2,05 and Tb 1,5, odds 2,50.
If exactly one goal is scored in the specified match, this result will fall under both outcomes, so the player who bet on them will receive their winnings twice. This is success in the middle.
In the case of Polish middles, the calculation scheme is somewhat more complicated (it is described in detail on the page describing Polish middles).