With OpenSSH v4 have been introduced a new feature, which “enables the sharing of multiple sessions over a single network connection“. The idea is the following: when a session with the remote host has been established, it opens a unix domain socket on your local machine. This session is marked as master. Other SSH sessions you initiate to the same remote machine may connect to the master session via the unix domain socket. The master SSH session then creates another interactive connection within the existing session, which your second SSH uses.
The advantage of this trick is that the connection setup is very much faster. Now there is no need to setup a TCP/IP connection, no need for key exchange, and overall no need for authentication.