Here we'll talk about how a person's body could also function as the bor, the obstacle in the public space. The Mishnah says that if you have two pot makers—pot makers would carry a very heavy burden on their backs—it was very common for them to fall. So the Mishnah uses them as the example. They were walking one behind the other in Reshut HaRabim, in the public area. The first one, because of the burden he was carrying, fell over and stumbled onto the ground. The second man, who was walking behind him, fell onto him. He was tripped up by the first man, who was still on the ground.
The first man is obligated to pay for the second man's injuries. He had the ability to pick himself up quickly, and he didn't. Therefore, the second man fell over him and hurt himself. Because he had the ability to remove himself from the floor—the obstacle on the floor—and didn't do it, he's liable for all the injury that happens as a result.
It must be that this Mishnah holds like Rabbi Yehuda in Mishnah Aleph. We had the machloket (dispute) in Mishnah Aleph over whether nitkal was poshe'a hu or nitkal lav poshe'a hu: if somebody falls, is that an act of negligence, or do accidents happen? We said Rabbi Yehuda's opinion over there was that stumbling is something that's unavoidable. If somebody falls onto the ground, it's not really their fault. They can't be held liable for that.
So, applying that to our Mishnah, the reason he wouldn't be obligated if the person fell over him right away is that he's falling over. The original man isn't responsible for his own fall. That happens. It's a fact of life. What he is responsible for is not getting up quickly, not taking care of himself. He left the stumbling block—which was himself, his body—on the floor, and therefore the second man stumbled over him.
So what we're seeing here is that since nitkal lav poshe'a hu, if somebody falls onto the ground, it's not his responsibility. He didn't do anything wrong. What is his responsibility? To get up quickly. Since he didn't get up quickly, he caused damage, and he's liable for that type of damage.