Bat dropping (sometimes abbreviated as BD) is a technique used primarily in the Home-Run Contest minigame to rack up very high damage on Sandbag. It is the act of dropping the bat over Sandbag, then hitting the bag with a move that does not send Sandbag flying, usually a meteor smash to hit the bag back into the ground, or some weak attack that negates the knockback of the bat drop.
Captain Falcon or Ganondorf's bat dropping technique is a classic example: they jump over the Sandbag, drop the bat, and then hit it with a down aerial. The bat knocks the bag up in midair and the spike immediately hits Sandbag back down; thus, Sandbag doesn't move. The bat should bounce off Sandbag, so one can jump and catch it again.
Not all characters have to use down aerial; examples are Mario's forward aerial, Yoshi's forward aerial, and Falco's up aerial. The timing for every character is different because of varying falling speed, attack speed, and jumping height.
 Example videos