The PRO Stealth Evo seatpost is Shimano's high-end carbon seatpost. In order to save every gram possible, the Stealth Evo takes a unique approach to the saddle clamp mechanism. Instead of the clamp bolting vertically to the post, it bolts down horizontally, clamping to the side of the seatpost. Because of this, the clamp actually grabs the saddle rails more evenly; instead of simply putting pressure vertically (top and bottom) on the rails, the PRO Stealth Evo clamps from the top, bottom, and inside the rail. This makes your saddle that much more secure.
The Stealth has a 20mm setback, and the post is made entirely of unidirectional carbon fibre. The top 3rd of the post is also aerodynamic, helping you cheat the wind a little. All this wouldn't amount to much though if the post wasn't actually very light, fortunately, a 27.2 weighs a scant 199 grams, helping you cheat the climbs a little also.