Twin tip skis are turned up in the front and the back. The twin tip design not only allows skiers to ski backwards, but it also alters the performance of the ski making it release from the turn easier than a ski with a flat tail.
Since the tails of twin tip skis are also upturned like the tips, the contact points where the skis naturally meet the snow are closer together. This shortens the effective edge of the skis making them seem shorter than they really are. Many skiers pick a twin tip ski that is about 5 centimeters longer than the size they would select for a traditional all mountain ski to compensate for the shortened effective edge.
For example, Jeff skis a 172cm All Mountain Ski but his Twin Tip Skis are 178cm long.
Additionally many of the twin tip skis sold on Skis.com are rockered. Camber is the unweighted natural arc shape of a ski in relation to the snow when the ski is laid on its base. Reverse camber or rocker is when the ski is bent up and away from the snow. Rocker shortens the effective length of the skis without reducing the physical length of the skis. Rockered skis provide additional control and ease in turn initiation, without reducing the length of the skis themselves. Since rockered skis seem shorter to the skier, it is common for skiers to purchase a longer length than they would if the ski isn’t rockered.