TSA has a scanner at most all stations, which looks like a small version of what you see at the supermarket. They just check your name, flight info against your ID. Also if the wifi or phone signal is poor, it might take a bit to pull up the barcode, because you get an email with a link after checking in. You press that link to get the bar coded boarding pass.
If you have the UA mobile app, a link to your boarding pass can appear on the home page of your mobile app profile.
I use it all the time. Just remember to have your phone charged up.