The 'D' button pressed downward can be used for several things...
1-While running in any direction, press the button down to belly flop upward.

2-Try holding the 'D' button down while having pikmin with you. (They carry you to the onion for a fireworks display and finally for Captain Olimar's invincibility move)

3-Around an enemy, press the 'D' button downward and watch them attack you in a mad frenzy (note: do nothing except change 'Z' button angle without pikmin for best results)