From the Super Mario Wiki
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When initiating the move, Waluigi would dribble the ball in a "W" shape, then a "Γ" symbol would appear. He would then shoot the ball toward the basket, a cyclone trailing behind it. Waluigi swims through the air and dunks it in.
The visuals are the same when shot outside the 3-point line, but as usual, a teammate needs to jump, catch the ball, and finish the dunk.