From the Super Mario Wiki
The Excite Spring is a special equippable accessory item that can be obtained in the Starbeans Cafe in Mario & Luigi: Superstar Saga and its remake. If players make a third type of unique coffee brew, directly after they have obtained the Bonus Ring, Professor E. Gadd shows up to drink the coffee and he gives Mario and Luigi the Excite Spring. When equipped, the Excite Spring can cause the equipped brother to float via holding a button when he is jumping to defend in battle, making evading enemy attacks easier.
The Excite Spring was originally intended to be given by an Excitebiker who would show up at the cafe instead of E. Gadd. While the character was replaced by E. Gadd, unlike all other Special Items, the item's sprite and name were both left intact.
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