English Profits on Polish middles, as well as on ordinary middles, occur as a result of successful bets on the event outcome in two (sometimes even more) different outcomes.
An example with a simple middle: there are two bets: Total sup. 0,5, odds 2,05 and Total inf. 1,5, odds 2,50.
In the event that one and only one goal is scored in the indicated match, this outcome will be winning in both cases, so the player who placed them will receive their winnings twice. This is called hitting the middle.
In the case of Polish middles, the calculation scheme is somewhat more complicated (you can view it in detail on the page describing Polish middles).