Hi Fabius It's actually quite a complicated answer. Dual density essentially allows the wheel to be softer, without compromising durability. Softer wheels loose less energy on rough surfaces, so they aren't slowed down by vibration and unevennesses in the road. the second part of that question has to do with deformation. When pushing you deform the wheel, storing energy in it, this effect is increased if you double push, because you transfer energy to the wheels via compression, which is released as speed, as you move from the outside edge to the inside edge. Joey mantia has a great video explaining double pushing and compression if you are interested. The core is also an important part of this, but alot of the speed comes from the wheels compressing and decompressing. - Kasper