Powder snow is made of very fine crystals with thin, delicate arms, which creates large cavities occupied by air. that's what makes it feel like skiing through a cloud. Those crystals are very delicate and are easily broken, which is why skiing tracked out powder isn't as nice as getting the first turns. Shoveling large volumes of snow, as you would for a BC booter, also breaks down the fragile arms of the snowflakes and creates more contact surfaces between crystals. with just a little bit of time, about a days time, the structure of the crystals along the contacts will reform, creating new bonds between crystals.
So, if you want to build a backcountry jump, shoveling the snow into a huge pile and leaving it overnight before shaping it will both remove pore space and create bonds between grains, leaving you with a dense, coherent pile of snow without the use of salt, water, or packing.
source: Building igloos in the backyard, avalanche training, and mineralogy classes