Does automatic direct connection to the host in the net still work?

At least a couple of years ago I recall that teamviewer hosts on the internet somehow detected whether it would be possible to open ports for incoming traffic. Don't know how that worked, but the bottomline is that after giving the tv ID and password of that system, I saw in the latter's network connections that a connection was established directly between the host and the controlling system. I believe that the host opened port(s) via UPnP and signalled somehow the controlling system to connect directly.

Do note that I'm not talking here about connecting to a system using it's ip address on the internet; connection is made the teamviewer way: ID and password.

Does this feature work and if it does, does it have some requirements (enable something in configuration, enable UPnP on the router or something else altogether)?