Polish middles are not so popular around the surebets and middle players, but they are very smart way to win money in long term. Some of you could recognize them as LAYS. This is not exactly the same, but in principle the way of winning is the same. As you know, in the normal middles you need a special outcome to happen to be a winner. In the polish middles you need an exact outcome NOT to happen to win. I will give you an example of value middle:
You can see a Polish Middle like following:
AH1 (-3/4) 2.25
X2 2.25
This example gives value of 2.50 and 12.50%. This means that if you think for this as an ordinary surebet, you will take 12.5% profit on whole stake you bet, if polish middle condition is met. The value 2.50 shows the proportions of what you could win to what you could lose. In this case the proportions are that you could win 1 but could lose 1.5 (1+1.5=2.50 the value)
So, the value of this PM is 2.50
You bet 200 on AH1 and 200 on X2. Your total investment is 400. Here you need to NOT happen EHX (-1) or home team to win exactly with one goal (1:0 2:1 3:2 etc.)
As you know the average EH1 (-1) odds 
in ordinary bookmakers are about 3.50 (could be more). In this case the fair odds should be 3.50 * 110% or 3.85. So it means that in long term if you bet on this odd in range for example 3.85 - 4.00 you will win nothing and will loose nothing (according bookmakers). In our case you play against the bookmakers odds, or you give a virtual odds of 2.5 for EH1 (-1) to the books. In average your virtual edge is about 54%. If you make a series of 385 middles which are very similar to the described one, you will have in average 285 Wins and 100 Loses which in digits looks like this:
in ordinary bookmakers are about 3.50 (could be more). In this case the fair odds should be 3.50 * 110% or 3.85. So it means that in long term if you bet on this odd in range for example 3.85 - 4.00 you will win nothing and will loose nothing (according bookmakers). In our case you play against the bookmakers odds, or you give a virtual odds of 2.5 for EH1 (-1) to the books. In average your virtual edge is about 54%. If you make a series of 385 middles which are very similar to the described one, you will have in average 285 Wins and 100 Loses which in digits looks like this:
From wins you win: 285 * 50 = 14 250
From loses you lost: 100 * 75 = -7 500
Total Profit: 14 250 - 7 500 = 6 750
Total Invested: 385 * 400= 154 000
TOTAL ROI%: (6 750 / 154 000) * 100 = 4.38% (this is more than you could do on average when you make surebets)
The described scenario is only hypothetical, because you will have very different types of middles with different outcomes that you need, different countries, teams etc. When you put PM you have to be sure that the odds you take is in your flavour, because for example if you have PM which need a draw to lose, and is from low series from Italy or France, you know that draw is very frequent outcome, so you have to take it, for example UNDER 2.2 odds. But in most cases VALUES UNDER 2.5 are good to be taken. Be aware - if decide to play with PM, it very dangerous, so do not start with big stakes. You could win a lot but could lose a lot too. If you are patient enough in long term you will see the result.