From the Super Mario Wiki
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The Green Switch is an Japan-exclusive e-Reader card that is used with the game Super Mario Advance 4: Super Mario Bros. 3. It was included in the Series 2 set of cards. It can only be used in World-e.
Once this card gets scanned, the timer and music slow down for the first 50 counts, giving the player more time to complete a level. When 50 counts are past, the "coin" sound is played and the timer and music go back to their original speeds.
Makes the timer go slow!