Twins are basically a flip tail that keeps going. Rocker is a flip tail that starts earlier, has a different curve, and keeps going. Any rising curve in the tail - compared to a classic square flat design - will allow more slip during the last part of a turn, so easier to release after the belly of the arc, easier to pivot or smear. That makes twins nice in bumps or tight paces like trees (or parks) where you can't always carve. OTOH, it gives them a slightly (or in your face) vague feel at the end that bothers former racers or skiers who like to carve a lot. Some knowledgeable folks like Sierra Jim feel that any flip tail has roughly the same effect on groomed as a true twin. Others, like me, disagree. In my case it's because I believe that the amount of rise and the point where the curve hits the flat seem to affect how the ski's side engages groomed crud or soft broken snow.
Any raised tail will also shorten the running length, which will mean at the same ski length, less stability at speed, more maneuverability. This will vary according to the ski design; Stockli and Volkl, for instance, make twins that are beefy and fairly stabile. Rossi, Armada, and Nordica make twins that are more park oriented, light, and less stabile. Finally, a really fat twin will have a different mission - backside freestyle more than park, so in my experience, fatter twins usually make better all-arounders. You get the idea, obviously everyone will have their own definitions of stabile and stiff, fat and thin. I own a couple of twins, never go into a park unless I'm chasing my kids. Twins IMO are great for lower speed hyper manuevering on variable smaller mountains. I've skied big mountains and steeps in one of them (Mojo 90's, Whistler and Mammoth), found it doable but not my first choice. Did not like them in chutes, although some do. So in a sense, twins are more versatile than any other ski - flat tails or rockers - just not as good at a set of things different skiers value. Like speed or grip with flats, serious pow or trees with rocker.
Finally, lots of us who don't hit the park still like to ski switch once in a while.
Edited by beyond - 11/19/10 at 7:17am