when a band goes crazy on stage in a beautiful way, usually out of some type of extreme emotion or energy they happen to be feeling.
Some good examples of bands that flip out are:
Big D And The Kids Table, At The Drive-In,
Arcade Fire,
Bad Brains, and A.W.K.
It can not only be characterized by frantic dancing, jump kicks, or guitar spins, but also by intense emotion conveyed to the audience through their music(good example being Arcade Fire).
A great example of a band that doesn't flip out is The Strokes. Their music may be good, but i think they care more about their 1500 dollar jackets and appearing cool to their super model girlfriends than putting on a good show. Those Pricks.