Muse were apparently asked to perform on Italian TV, but they weren't allowed to play live, and had to settle for playback. It's a well-known fact that Muse *hates* to do this, so they had some fun with it instead. Everyone switched instruments and completely made a mockery of the whole show.
Drummer Dom Howard pretends to play bass and sing
Vocalist/guitarist/keyboardist Matt Bellamy pretends to play drums (with hilarious results)
Bassist Christopher Wolstenholme pretends to play guitar and keyboards
They keep it going through the interview as well, with Dominic still pretending to be the frontman of the band :-)
Watch the "performance" here: