They work by tapping your NFC capable phone to the back of the tag which then triggers a NFC application on your phone. I chose this one:

You then tell the app which actions to take for the given tag. When I get to work, I want my WiFi off, Bluetooth Off, and Ringer on Vibrate. You can also sent texts or emails, or even call a custom Tasker task.

The tag is sensed almost immediately. I stuck my office tag to the bottom of the tray where I put my phone. So far it works slick.