Dropstepping is when you lunge forward and down into a shot. Not necessarily punching down. I've seen people dropstepping into huge shots where the pads have been placed above their own head height. Sorry to mention dempsey again but he dropstepped in to a lot of his shots against Jess Willard who towered over him.

As you throw the shot move your front foot forward and drop the weight down on to it transferring your bodweight in to the shot. As Adam says its great for straight shots, also works well for right crosses, well it has when ive trained them. Couple it with correct body mechanics, and hip movement and it can create a really powerful shot.