SSH Reverse Tunnel

The SSH has many advance usages. The reverse tunnel is a good example and a very useful one. It answers the question: if a user can ssh from a server A to a server B, can the user ssh back from B to A, even A is behind a router? In my case, it is very helpful. I have a desktop in the office, but the office VPN has not been set up yet. How can I ssh to my office desktop from home? The answer is SSH reverse tunnel, using a server with a public IP as a jumper. Followings show how did I do it.