Demai, Chapter 7, Mishnah 2. The previous mishnah described how a person may eat food on Shabbos that belongs to someone who is not careful about separating tithes: by designating the tithes in advance, before Shabbos, and making the appropriate declarations, even though the food is not in his possession.
Our mishnah describes a similar procedure, except that it is performed on Shabbos itself. The commentators explain that our mishnah is in addition to the procedure of the previous mishnah: besides making the declaration on Friday afternoon, one must repeat it in practice when the food is actually before him.
There is another difference between the two mishnayos: our mishnah describes a case of a beverage, whereas the previous mishnah apparently dealt with food.
The words of the mishnah:
"Mozgin lo es hakos" - a little wine is poured for him on Shabbos. And what does he say?
"Mah sheshiyarti b'shulei hakos harei hu ma'aser" - what I will leave over at the bottom of the cup shall be tithe, and its measure is one hundredth of the total amount, as we will see.
"Ush'ar ma'aser samuch lo" - the rest of the tithe is adjacent to that portion which was separated at the outset.
"Zeh she'asisi ma'aser asui terumas ma'aser alav" - the portion that was separated at the outset as tithe, in the amount of one hundredth, shall be the tithe of the tithe for the rest of the tithe that he declared to be adjacent to it.
"Uma'aser sheni b'fiv, um'chulal al hama'os" - the second tithe is at the top of the cup, and it is redeemed onto the money that was set aside, and from now on he may drink.
The question of the Yerushalmi:
The Yerushalmi asks: when does the designation take effect? Neither possibility works out:
If it takes effect immediately, at the time of the first declaration before he drinks, then the tithe of the tithe is mixed into the rest of the beverage that he wants to drink, and this is called 'meduma'. Meduma is a mixture of Terumah and ordinary produce together, and it may not be drunk. If so, there is no way to make it the tithe of the tithe before drinking.
And if it takes effect only after he drinks, then at the moment he drank, the tithe and the tithe of the tithe had not yet been separated from it, and it turns out that he is drinking tevel.
The answer of the Yerushalmi:
The Yerushalmi explains that this is a case in which he says: "Me'achshav ulich'she'eshteh" - that the designation should take effect now, at the time that he drinks. In other words, at the moment of drinking, the designation takes effect retroactively. Through this retroactive declaration he is able to drink at that very moment, when the beverage is no longer tevel, since the tithe of the tithe was separated from it with earlier force.
In summary: in this way one may drink on Shabbos a beverage from which the tithe of the tithe has been separated, without it being tevel.