Inside the SlingBox there is hardware to digitize the audio/video output of the connected device, send the digitized stream over the ethernet to a PC running a client that can decode the stream, and simulate the remote control for the connected device by using the infrared blaster in response to buttons pushed on the user interface presented on the PC client.
In addition, to accessing the stream from a PC on the local ethernet, it is possible to connect to the device from anywhere in the world over the Internet.
It was very easy to install and worked perfectly the first time I tried it out. The net result is that anyone I authorize (including me) can watch programs recorded on my TiVo as well as control the TiVo using the TiVo interface from anywhere in the world. Impressive device. Here are some screen shots of what I see on my PC.