The manufacturer says this about the Booster Straps:
The strap worn by Bode Miller is now used by a great many athletes of all skill levels … for instant ski response. Many will have noticed the strap appearing out of the suit of many athletes, including Bode Miller. The article in question is Booster, an essential accessory for optimising ski boot performance. When used correctly, it guarantees maximum ski boot performance and therefore extraordinary ski control, improving comfort and completely eliminating the empty space between the boot tongue and shin, thus serving a shock-absorber function and aiding the prevention of shin bang and other boot maladies. The Booster Strap is made of high strenght elastomeric webbing with an anti-slip micro-adjustable cam buckle. The elastomer allows flexibility, vibration and shock damping. The cam buckle assures a better fit and more comfort. The Booster Strap elastically fastens together the leg and the ski boot and provides direct ski control so the skis will start turning sooner and and will end the turn faster. The graded elasticity of the product allows skiers to choose the reaction speeds that best suit their technical and physical characteristics and skiing ability.