Chullin, Chapter 2, Mishnah 9. This chapter has been examining the ways in which an otherwise proper shechitah can be disqualified, or forbidden, because of what it looks like and what stands behind it. Our mishnah takes up a case where nothing is wrong with the knife or the cut, but the setting itself gives the impression that the animal is being slaughtered for avodah zarah.
Not Into the Sea, and Not Into Rivers
The ruling is stated first and the reasoning follows. "Ein shochatin," we do not slaughter, and the mishnah then lists where: "letoch yamim," into seas, "velo letoch neharot," and not into rivers. What is the objection? Someone passing by would read the scene as a sacrifice offered to the spirit, the angel as it were, appointed over those waters. The slaughterer intends nothing of the kind, yet the picture he creates speaks for itself.
Not Into Vessels
"Velo letoch kelim," nor may one slaughter directly into utensils. Here the suspicion runs in a different direction: it looks as though the person is gathering the blood in a container in order to sprinkle it afterward on an altar of idolatry.
What Is Permitted: Murky Water and a Boat
Now the mishnah tells us what is in fact allowed. "Aval shochet hu," a person may indeed slaughter, "letoch ugah shel mayim," into a hollow of water. Blood that has mixed into standing water is no longer fit to be sprinkled on any altar of avodah zarah, and with that possibility gone, the onlooker has nothing to suspect.
"Uv'sefinah al gabei kelim," and on a boat one may slaughter over vessels. This refers to utensils turned upside down: the blood falls on their outer surface, runs off the sides, spills onto the deck, and from there flows into the sea. The blood does reach the water, but only indirectly. Anyone watching understands exactly what is happening: the person is simply keeping his boat from being soiled with blood. There is nothing here that looks like homage to the spirit of the sea.
The Pit
The Torah instructs us, "uvechukoteihem lo telechu," do not walk in the practices of the Canaanite idolaters. One of their customs was to slaughter an animal beside a pit and then hold their meal on the spot.
Therefore: "ein shochatin legumah kol ikar," one may not slaughter into a pit under any circumstances whatsoever. This holds even inside one's own home, and even when the whole purpose is nothing more than keeping the house clean. Draining the blood straight into a hole in the ground is not an option.
What may be done is this: "aval oseh gumah betoch beito," a person may make a hollow inside his own house, "bishvil sheyikanes hadam letochah," so that the blood should enter it. He performs the shechitah on the floor beside the hollow and not above it, and the blood finds its own way in. Because it arrives by an indirect path, the arrangement is permitted.
"Uvashuk lo ya'aseh chen," in the marketplace, however, one may not do even this. Not even the indirect arrangement is allowed there, "shelo ychakeh et haminin," so as not to lend encouragement to the heretics, the non-believers, who would see the scene and imagine that something resembling idolatrous worship is taking place. In a public setting, even letting the blood flow into the pit indirectly is forbidden.