From Playstation Insider:
"First of I must say that this game nails the feel of Rock Band on the consoles. For all intents and purposes, Rock Band Unplugged is exactly what it sounds like. It's Rock Band but without the instruments. Unplugged lets you juggle all four instruments at the same time. You'll play a section of the track, called a phrase, and if you nail it to a good degree of accuracy the track will play on it's own for while. You'll then jump onto another instrument to keep that one sustained and you repeat this process for all the guitar, bass, vocals and drums tracks until the song is over.