From the Super Mario Wiki
When used, Mario will head away from the enemies, and Dreamy Luigi will release Luiginoids that form a compact group with Dreamy Luigi that carries Mario towards the enemies. As Mario and the Luiginoids approach more compact groups of Luiginoids en route, the player must press to make the Luiginoids jump on to the incoming huddles. The player while in mid-air can tilt to redirect the current pile of Luiginoids and help pile them on to the stationary Luiginoids in a more firm fashion.
If the player makes the Luiginoid Stack become too out of balance, Luiginoids may fall off the stack. Once the group reaches the enemies, the stack of Luiginoids will spring into the air to land on the target enemy with all wobbly parts of the stack falling off just before this happens. Upon the stack's impact on the target, the player must press to make all Luiginoids present in the stack rain on to all the enemies afterwards. The more Luiginoids stacked at the end, the more damage dealt. After the Luiginoids finish raining, the player can press right when Mario lands on the originally targeted enemy for bonus damage.
Jump with the B Button and stack up Luiginoids as straight as possible! While jumping, adjust the landing zone with left/right on the circle pad! Press the B Button as the stack lands. Use the A Button at the end for Mario.
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