Sentence:  Unlike powder, <WSD>hardpack</WSD> permits great speed (no spiny arms on those balls of ice to drag on your ski bottom). Where powder is sluggish, crudely transmitting your body movements to the snow, <WSD>hardpack</WSD> is the snow of precision. 
Meaning of the word marked with <WSD> tags:  A material (such as dirt, gravel, or snow) compressed into a hard, smooth surface for roads etc.