1:
In order to understand our present mishnah we must bear in mind the two kinds of yeyn nesekh which we have mentioned from time to time. In the age of the Tannaïm and the Amoraïm yeyn nesekh proper refers to wine which is known to have been used in some way or other in an idolatrous ritual (or which may be assumed to have been thus used beyond reasonable doubt). Wine belonging to non-Jews (or which had been touched by them) about which it is known that they did not use it for idolatrous purposes (or when such an assumption may be safely made) is referred to as stam yeynam.

2:
It is forbidden to derive any kind of benefit from yeyn nesekh; this we have learned in many of the mishnahs that we have studied in this tractate. While the sages forbade a Jew to drink stam yeynam they were not so demanding as regards deriving some kind of benefit from it.

3:
Our present mishnah describes a situation in which some non-Jewish wine fell (or was poured) into the vat of a Jewish winepress. In AZ 057 I described the physical arrangements of a winepress:

The harvested grapes were dumped into the winepress. This was a cemented cistern or pit, usually dug out of ground rock for the purpose. Once in the winepress the grapes were there trodden to extract their juice. Every now and then someone would move some of the trodden grapes onto a pile of mashed grapes situated at the far end of the winepress. From there a conduit led to a second cistern called the vat. The vat was always lower than the winepress so that the juice oozing from the trodden grapes in the pile could run down to be collected in the vat.

(In the original shiur there is a picture of an ancient winepress.)

4:
We have learned many a time and oft that when in a mishnah an anonymous teaching (stam mishnah) is followed by a teaching ascribed to a named sage the purpose of the format is to let us know that the halakhah follows stam mishnah (the majority of the sages). However, in our present mishnah that is not the case. The Gemara [AZ 74a] clarifies that Tanna Kamma is referring to a situation where the intrusive wine was definitely yeyn nesekh, while Rabban Shim'on ben Gamli'el is referring to a situation where the intrusive wine is stam yeynam.

5:
Thus, if stam yeynam gets mixed up with kosher wine it is not forbidden to derive benefit from it (though it is forbidden to consume it). This applies not only to wine in an open vat but also to casks of wine that got mixed up and the kosher casks cannot be distinguished from the casks containing stam yeynam.

6:
So, if stam yeynam gets mixed with kosher wine, while Jews may not drink it, the owner has not entirely lost the value of his product. He may derive benefit from it to the extent that he may sell all the wine in the vat (or in the mixed casks) to a non-Jew. However, he must deduct from the agreed price the value of stam yeynam that was involved.

In AZ 071, in the Discussion section, I responded to a message from Oren Steinitz, who had quoted a responsum of Rabbi Israel Silverman. Rabbi Silverman had said that it was permitted nowadays to use stam yeynam for secular purposes but kosher wine should still be preferred for the purposes of a mitzvah such as kiddush or havdalah. I wrote:

I really do not understand what the psychology of using kosher wines in religious ceremonies is all about. Either something is permitted or it is forbidden. If stam yeynam is permitted then it is permitted also for Kiddush and Havdalah; if it is forbidden it is forbidden for secular use as well.

Michael Lewyn does not agree:

I am not sure I agree with your response, and here's why: at religious ceremonies such as kiddush and havdalah, you may have guests who view the issue more strictly than you do. So even if you do not limit yourself to kosher wine generally, you may wish to do so for kiddush and havdalah for the sake of respecting their sensitivities.

I respond:

In other words the host recognizes the legitimacy of the view of his guests. In which case, how can he permit himself the lenient view? Again, I repeat: either something is permitted or it is not. In normal circumstances Halakhah does not recognize a situation in which something is permitted to me but not to you. If the host, in Michael's example, truly believed that stam yeynam is permitted he could have used it himself while providing kosher wine for the sensitivities of his guests. But that is not what Rabbi Silverman said. He required kosher wine for everybody for these ceremonies "for psychological reasons". I must assume that what he meant was that it is "nice" to use kosher wine when fulfilling a mitzvah. I do not understand that kind of psychology. There are situations in which Conservative Judaism recognizes two alternatives legitimacies: but this is not what Rabbi Silverman says. He requires only kosher wine for ritual purposes "for psychological reasons". I do not understand his halakhic reasoning.