Say me and another identical computer are together in a local network (I am 192.168.1.5 or A, and he is 192.168.1.6 or B, for example) behind NAT (say our public IP is 22.214.171.124), and we have a torrent client running. We both have a certain file, let's call it "file.txt". An outside computer (let's call him C) wants to get a piece of the file. So he tries to make a connection (or rather requests the file or something, I'm not so sure how the protocol works) to 126.96.36.199 right?
Now how does NAT know to which one of us (A or B) the packet is supposed to go? I must stress (if it wasn't clear enough already) that the outside packet (coming from C) is the first ever packet from that address, which means that neither me nor B know of the existence of C.