Tunnel Tab (Login Dialog)
Tunnel tab allows you to setup your session to tunnel through SSH connection.
You need to check Advanced options to reveal the tab.
To enable tunneling, tick the Connect through SSH tunnel.
Host to setup tunnel on box configure basic settings for the SSH tunnel session. The meaning of the boxes is the same as the respective options for the main session on the Session tab.
Use Local tunnel port if you need to use specific port for the SSH tunnel. This is particularly useful if you want to use the opened tunnel for another applications. When Autoselect is selected, WinSCP will automatically pick a free port from range 50000 through 50099.
1) See FAQ for help.