From the Super Mario Wiki
Lakitu Launch is a minigame in Super Mario 64 DS and New Super Mario Bros.. It is one of Wario's minigames in Super Mario 64 DS and Action minigame in New Super Mario Bros. The player must use the stylus to slingshot slingshot in the giant Green Shells the Lakitus are carrying. At the end, the Spinies will jump out of the shells and will be counted.
In multiplayer, the player must sling the Spinies and hit the Lakitus that have stars to earn more points than their opponent. If the Lakitu is holding a star, it should only be hit if it is on the lower screen.
 In-game instructions
New Super Mario Bros.: Shoot as many Spinies as you can into the tubs within the allotted time.