a hole that closely fits the baseplate of a truck is cut through the deck, the baseplate the truck is removed from the hanger, then both are reattached with the deck between. the bolts are used upside-down for this.
trucks are dropped to lower the riding surface for stability in speedboarding and sliding, and to give a more comfortable push for cruising. the closer the deck surface is to the ground, the less the distance one must step down to push is.
also, the dropped deck will cradle more than tip than a conventional or risered one. this causes a dropped deck poorer geometry for traction because it centers the rider's weight toward the wheels outside of a turn and gives a strange feeling while carving.
if the riding surface is below the axle, the board is probably TOO low.