This feature has 3 limitations: it's US-only, you need a smart TV and the phone/tablet needs to be on the same WiFi network as the TV. The first limitation makes sense, but the other two seem unnecessary. After all, Google's app can't communicate with the TV and the TV doesn't broadcast the program you're watching.
Here's Google's explanation: "Your Android device uses the Simple Service Discovery Protocol (SSDP) to determine the presence of a TV on the network and related information, such as its model and brand name. This is a standard way for networked devices to locate other devices or services on a home network." So a smart TV is required to only show the TV card when you're watching TV.
"To provide relevant information after you touch Listen for a TV show, TV cards rely on both the WiFi signal and the sound from your TV. You may need to turn up the volume for the cards to work correctly," explains Google. Does this mean that Google records all the TV shows from the US channels?
"Other cards may also appear, telling you more about specific show content as it happens. For example, they may give you more information about celebrities, actors, or other people mentioned in the show."
This is a cool feature, but I don't see why it wasn't added to Google Sound Search. In addition to finding songs, the widget could also find TV shows. Even better, this could be added to Google Voice Search.
Probably not many people remember this, but Google Video started back in 2005 as a search engine for TV content. "Google Video will search the closed captioning text of all the programs in our archive for relevant results. Click on a program title on your results page and you can look through short snippets of the text along with still images from the show," suggested Google at that time. Ironically, Google Video didn't host any video at launch, but this changed after a few months, when Google Video became a video-sharing site.