Replying to Underflips, WTF are they

I was reading some stuff about skiin and whenever someone says so and so tossed an underflip i'm out in the cold, this is like the only trick i'm not familiar w/ and it's kinda bugging me. Does anyone know what they are/ how to do one?

McConkey: 'What the hell are these things?! What are you a mountaineer of something. He's got ropes & shit hanging off him. Haha!' - about Dean Cummings

McConkey: 'The polaroid doesn't help at all, I took it of the wrong face.'
Dean Cummings: 'Thats what makes you a pro Shane, not knowing.'
MC: 'Just ski down there and jump off something for crying out loud!' - Shane about to 1st descent