After using the Polartec® Power Stretch fabric when hiking in the Dolomites, Castelli thought this could be an excellent fabric in cycling. It's luxuriously thick and warm, and the exceptionally tight knit of the fabric keeps a lot of the wind out, yet it lets enough air come through to prevent overheating and keep you dry. They have worked with Polartec® to give it the Hard Face finishing for extra durability, and a DWR finish to protect from light rain. The result is a product that's never been available in cycling. Far warmer than a traditional jersey and far better breathability than any jacket with a windproof membrane. Perfect for those spring and fall rides where you need insulation from the crisp, cool air, but you're not ready for a sweat fest inside a jacket. It's a new way to dress for cycling.