Here's the story.
When I run Tor software, in theory my computer becomes a relay, so user x sends some data to my computer and my computer passes that data to user y and vice versa.
This is achieved by user x calling my IP address (for example 188.8.131.52) and sending something to the Tor software running on my system. But it's not possible! My ADSL router doesn't allow traffics to go directly to my local IP address (for example 192.168.1.5). Instead it shows an error page or something (default firewall policies of all known brands!)
And if so, how does my router knows which local IP address to pass the incoming data?