FWIW, none of this made any sense to me until I mentally ditched the concept of "tunnels." The metaphor was too physical for me?
Personally, the metaphor that made it work for me was "postcards going to a motel." As in AHA, when I'm doing SSH Tunnelling, Room 22 is the only one open and receiving mail, all the other rooms are locked. Now, there might be a website behind the door on locked room 443, and I can send my postcards to room 22 with the instructions to send them on over to 443 once they get there, and over at the motel they'll also know to do the reverse.
Personally, the metaphor that made it work for me was "postcards going to a motel." As in AHA, when I'm doing SSH Tunnelling, Room 22 is the only one open and receiving mail, all the other rooms are locked. Now, there might be a website behind the door on locked room 443, and I can send my postcards to room 22 with the instructions to send them on over to 443 once they get there, and over at the motel they'll also know to do the reverse.