Here, the Mishnah will talk about that looks like it's being done for idol worship purposes. The Mishnah says a person is not allowed to slaughter directly into the sea or into rivers because it looks like a person is offering a korban (sacrifice) to the angel of the sea or the river. He is also not allowed to slaughter directly into utensils because it looks as though he is trying to collect the blood and then sprinkle it onto a Mizbeach (altar) of idol worship. But he is allowed to slaughter directly into a pool of water because once the blood is mixed with water, it is not fit for sprinkling onto the Mizbeach, so there is no concern about doing it there. If he is on a boat, he is allowed to slaughter on top of vessels or utensils that are flipped upside down, so that when the blood lands on them, it drips off the side of the utensil, onto the side of the boat, and then into the sea. He is causing the blood to fall into the water, into the sea, in an indirect manner, so when people see that, they are not going to suspect him of worshipping the angel of the sea. They will realize he is just doing it to avoid getting blood into his boat.
The Pasuk (verse) tells us you shouldn't follow the ways of the Kena'ani idol worshippers. It was the practice of idol worshippers to slaughter an animal near a pit and then eat a meal right there. You are not allowed to slaughter an animal so that the blood flows directly into a hole under any circumstances. That means even if a person is doing it in his house, he is not allowed to do it, even though his intention is only to make sure the house does not get dirty. You are not allowed to drain the blood directly into a pit, directly into a hole. But you are allowed to make a hole in your house and slaughter over the ground near the hole, not directly in the hole, so that the blood enters indirectly. But if you are in the marketplace, you are not allowed to do that. You are not even allowed to let it drain indirectly into that hole, so that you do not give support to the heretics, the non-believers, who might think that something is going on that looks like idol worship. You cannot even indirectly drain it into the hole.