1:
Our mishnah is concerned with three different situations. (These situations, in the Gemara, are dealt with as separate mishnahs.) The first situation is one where a non-Jew wishes to pay his Jewish workers in wine instead of in cash. The second situation is when a Jew is bargaining with a non-Jewish customer about the price of wine to be sold. The third situation is when a non-Jew pours wine into a Jew's cup. These three situations are followed by a general rule concerning liquids.

2:
In the first situation we must imagine that some Jews have done work for a non-Jewish employer. This employer, instead of giving them money as their wage sends them a cask of wine (which is, of course, yeyn nesekh). Such a situation should not surprise us since in earlier times payment in kind rather than in cash was quite accepted and sometimes even expected. In this situation the problem of the Jews is, of course, that because the wine is yeyn nesekh it is useless to them. Our mishnah rules that if they object to this arrangement before taking possession of the wine they are entitled to demand payment in cash and not in kind. However, if for any reason that have taken possession of the wine before they realise their mistake they have lost their wage. This is because they are forbidden to derive any benefit from the wine which is yeyn nesekh and the moment that they took possession of it it became legally theirs. Since the yeyn nesekh now belongs to them they cannot give it back to the employer in exchange for money because they would thus be deriving benefit from the forbidden wine. (This is another example of the fact that the onus is entirely on the shoulders of the Jew and the non-Jew is in no way involved in this matter.)

3:
The second situation is similar to the first in its essential ruling even though it is different in its details. If a non-Jewish customer agrees with a Jewish vintner the cost of some wine that he wishes to buy and the Jew measures out the wine for the non-Jew the Jew is now permitted to receive the payment: since the sale was in fact effected from the moment that the non-Jew settled the price: the Jew can accept payment because when the deal was done the wine was kosher. But, if the Jewish salesman measured out the wine before he agreed the price with his non-Jewish customer he may not accept payment since the wine has become yeyn nesekh from the moment is was passed over to the non-Jew. (Here is yet another example of how the onus devolves entirely on the Jew and the non-Jew is in no way involved.)

4:
The next situation is similar. In this situation a Jewish salesman measures out wine for a non-Jewish customer using a funnel to pour the wine from the cask into the non-Jew's container. The ruling is that if there remained in the funnel dregs of wine and the Jewish vintner now uses the same funnel to pour out wine for a Jewish customer the wine sold to the Jewish customer has become yeyn nesekh!

5:
In order to understand this ruling we must recall something that we learned when studying Tractate Yadayyim (chapter 4, mishnah 7). Here is what I wrote at that time:

Thus, in the situation described in our present mishnah, the stream of wine connected the funnel with the wine in the non-Jew's flask which was now yeyn nesekh. Thus remains of the wine in the funnel are also now yeyn nesekh. When the hapless vintner uses that same funnel to measure out wine for a Jewish customer the remnants of the wine in the funnel render the wine measured out for the Jew yeyn nesekh.

6:
Our explanation of this third situation has also explained the general rule which is the last clause of our mishnah.

You may recall that in AZ 067 I responded to Israel Man concerning his claim that the laws of yeyn nesekh are derogatory towards non-Jews. I invited participants to offer their comments as well. Here is a message in this matter which was sent to me by Ronen Lautman:

I would like to add to the discussion about Jews taking special care of their wine.

Israel Man wrote: "Imagine that a Gentile organization would come out with a printed booklet containing the AZ mishnaiot with only one change i.e. swapping the word Goy or Nochri with the word Jew and vice versa. We would have cried GEVALT! Antisemites!"

I don't believe that this is the case. There are plenty of groups that guard things that are precious to them without scandal. For example, Catholic Christians hold by the concept of transubstantiation (the bread and wine of the Eucharist transforming into the flesh and blood of Jesus), while Protestants do not hold by this idea. Catholics have ruled that no non-Catholic may celebrate a Catholic Eucharist, and that no Catholic may celebrate a Eucharist that has not undergone transubstantiation. If a Catholic priest were to admonish his congregation against eating crackers distributed by Jews or Protestants for religious purposes, there would not be a large outcry of "antisemitism!"

I respond:

It seems to me that Ronen's example is most apposite because he has restored the discussion to the realm of religious observance.