piggy back是什么意思？

遇到这么一句话。

 　　// Connect the Socket.IO framework up to the web server to piggy back on its connection
 　　// 把Socket.IO框架连挂在网页服务器以依附到它的连接上。
    
    io = socketIo.listen(server),

 先看看网上搜的参考资料：

（一）

You need to use a fast connection everywhere. The new iPad is the first Apple tablet that can connect to 4G networks. (You can buy a new iPad that works on either AT&T's 4G network or Verizon's 4G network.) If you go with Verizon, you can also use the iPad as a hotspot, allowing other devices to piggyback on its wireless connection. And Apple now sells only the Wi-Fi version of the iPad 2, so if you need an anywhere connection, the new iPad is your only option among Apple tablets.

意思明白了：依附、搭顺风车

（二） 

Wireless installation is for wireless CCTV wherein a dedicated server or connection host acts as a relay point for all the information being broadcasted by the camera. You cannot place these cameras all over the building and expect them to store images on their own. If used in a residential area, you can piggyback on your Internet so that your wireless system will have its own connection. However this is risky because if your Internet connection disappears, your system is good as useless.

（三）

出处：http://select.yeeyan.org/view/213582/212487