From the Super Mario Wiki
The Orange Switch is an e-Reader card for use with Super Mario Advance 4: Super Mario Bros. 3. It was included in the Series 1 set of cards. Although technically a Power-Up card, the Orange Switch is labelled as a "Game Level/New Level" card.
Once scanned, any non-boss enemies that Fire Mario defeats with a fireball will turn into a coin, much like in Super Mario World. Compared to the other switches from Super Mario Advance 4: Super Mario Bros. 3, the effect of the Orange Switch is quite minor.
When Mario defeats an enemy using the fire attack, it turns into a Coin.
Enemies turn into coins with a fireball!