Shabbos, Chapter 13, Mishnah 7 - the final mishnah dealing with the melachah of trapping. This mishnah addresses the case discussed in the previous mishnah: a deer that entered a house on its own, which is not considered trapped until the door is shut or the entrance is blocked. Here the blocking is not done with a door, but by a person who sits in the entrance and blocks it with his body, in such a way that the deer cannot get out.
The first case - an entrance that one person does not fill:
This refers to a wide entrance, where the body of a single person is not enough to block it, and the deer can still get out. A second person comes and sits down beside the first, and with that the entrance is filled and the deer can no longer get out. The ruling is that the second person is liable: the first did not trap the deer, since at the time he sat down the deer could still get out, whereas the second, who completed the blocking of the entrance, is the one liable for trapping.
The second case - the first person filled the entrance:
The first person sat in the entrance and filled it entirely, and afterward the second came and sat down beside him. In this case the first person already trapped the deer by his action. And even if the first got up and left, and the second is the one who continues to maintain the blockage, this is the ruling:
The first is liable: he is the one who filled the entrance from the outset, and with that act the trapping was accomplished, and even though he got up and left, the liability remains upon him.
The second is exempt: he did not trap the deer at all. All he did was continue sitting in his place, and the deer remained in the state it was already in - trapped and standing trapped.
"Lemah hadavar domeh?" - To what may this be compared:
The mishnah compares this to someone who locks his house in order to guard it, and a deer turns out to be secured inside. The Ran explains the end of the mishnah: the deer is already caught and the door is already closed, and all that the one who locks it does is provide additional protection, extra safeguarding - and there is no new act of trapping here.
In summary: we have learned two cases of blocking an entrance with a person's body: at an entrance that one person does not fill - the second, who completes the blockage, is the one liable; and at an entrance that the first filled entirely - the first is liable even if he got up and left, and the second is exempt. And this is the ruling that emerges from the comparison at the end of the mishnah: the entrance was already blocked, and the one sitting there is merely continuing an existing state, and therefore he is exempt, since the trapping already took place beforehand.