They throw everything in to keep you warm without bulk, a combo of Primaloft and Gore Windstopper and even neoprene are used to keep bulk down and warmth up.
These mittens are warm enough to go for a hike on a sunny 20-30°F day without getting cold hands. But with a slight cost, if you build up some steam and start to sweat they are not very breathable. The other slight nit to pick is that the cuff is really stiff and hits my watch. But only a slight nitpick.
They are warm enough for me to wear on cold days but thin enough to still handle dog leashes and use camera buttons. But on super cold days you would need to pair them with a mitten cover or I wear them under my Dogwood Design Bar Mitts on single digit days. I like them and if you are looking for an all in one glove for cold but not freezing cold days pick these up.
Swix says: http://www.swixsport.com/Products/Apparel/Gloves-Mittens/Icon-mitts-Mens
Gore Windstopper Outershell with neoprene flex panel and gold reflective piping with contrast stitching. Chamude palm and neoprene cuff with Velcro closure tab. Thumb reinforcement with SWIX Shock-Eze impact protection zone on palm. SWIX-Dry lining. Nosewipe thumb back. Hydra-Vent breathable performance. Primaloft insulation.-Gore Windstopper outershell with Neoprene flex panel. SWIX-Dry lining.-SWIX Shock-Eze impact protection zone on palm.-Hydra-VENT breathable performance.-Primaloft insulation.-Chamude palm and fingers with thumb reinforcement. Fabric Content: 100% Woven Fabric