Why/How does the Pilatus PC6 Approach nose-down?
How: With flaps out and the engine at idle that is the pitch for this aircraft that will yield its maximum decent speed. 

Why: Max rate decent are commonly trained to be used in emergency situations but in this case its likely being used to save time since jump planes make money jumping. In most situations passenger comfort and airspace organization limits decent rates and keeps them controlled. Jump planes simply serve the purpose of getting passengers to jumping altitude and then getting back on the ground to scoop up the next load so max decent is used to save time.