Machshirin, Chapter 5, Mishnah 9. This mishnah steps away from the question of hechsher and teaches a fundamental rule about pouring liquids: does tumah travel upward along a stream? The mishnah states: "Kol hanitzok tahor, chutz mid'vash haziphim v'hatzapachas." Every poured stream is tahor, except for Ziphite honey and honey batter.
The Rav lays out the scenario. Someone has in his hand a vessel that is tahor containing water, and he tips it so that the water runs down into a second vessel that carries tumah. For as long as he pours, an unbroken column of liquid joins the vessel above to the vessel below. Should we rule that the tumah of the receiving vessel travels up along that column and defiles what remains in the vessel above? To this the mishnah answers no. Motion here runs one way only, from the higher vessel toward the lower, and a flow that descends is not regarded as doubling back to carry tumah in the opposite direction. Whatever is still in the upper, tahor vessel keeps its taharah.
That is the rule for liquids in general. There are, however, liquids that are thick and sticky enough to have a certain body and structure of their own, and for them the halachah is different. With such a liquid, if a person interrupts his pouring in the middle, the flow does not simply break off; it draws back up whatever has already gone down. Since the poured substance and the substance in the upper vessel really do function as one connected mass, in these cases we do say that the tumah rises.
The mishnah gives two examples. The first is d'vash haziphim, Ziphite honey; the Rav explains that there was a city called Ziph, and this particular type of honey came from there. The second is tzapachas, which the Rav explains as a kind of batter made of honey mixed with flour. Both of these, when the pouring stops, pull back what was poured, and therefore the stream is a connection through which tumah passes up into the upper kli.
The mishnah continues: "Beis Shammai omrim: af hamikpeh shel gerisin v'shel pol, mipnei shehi soledes l'achareha." Beis Shammai add to the list the thick porridge made of split beans and the one made of whole beans, because it springs back on itself. This porridge is somewhat thinner than honey, and the Tanna Kamma held that it does not possess that quality; Beis Shammai maintained that it does. Their argument, then, is simply about whether this degree of thickness is sufficient.
Notice, though, that both sides agree completely on the criterion. The test is soledes l'achareha: if the pouring is halted and the liquid pulls itself back, returning to its original mass, then the stream is a genuine connection and it carries tumah upward. Where the substance lacks that quality, kol hanitzok tahor, and the upper vessel remains untouched.