From the Super Mario Wiki
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Once this card is scanned on the e-Reader, it makes all non-lethal damage power-down the player to Small form (as in Super Mario Bros. and its derivatives, the Japanese version of Super Mario Bros. 3, and the original version of Super Mario World), replaces most of the enemies with more difficult ones (similar to the Hard Mode of Super Mario Bros.), and doubles the bosses' health. However, the points that the player receives are doubled, making the player's score accumulate much faster.
Enemy Power Up! Score X2!!