It's when you see someone dancing and singing along to a song but you can't hear them singing. Usually done at traffic lights when people don't know that they're being watched. It's lipsyncing in reverse. Named after a performer who is renowned lipsyncing and dancing to her songs.
I was stopped at the traffic lights and this chic pulled up behind me. I looked in the rearview and she was Doin' the Britney, She was grooving about, waving her hands and singing to a song. I couldn't help but have a good laugh.