From the Super Mario Wiki
When using this special technique, Mario and Luigi jump into a window, which teleports them close to a random opponent. The windows will appear at different orientations depending on which enemy is the target. As Mario or Luigi jump out of the window, the player has to decide which button to press: , , or both of them. If the player fails a button press, the attack will end. This attack will cost eighteen SP to use (nine if reduced).
This is unlocked by defeating the optional Shroob boss. The shape of the window itself seems to be modeled after the Shroob Empire symbol used in Mario & Luigi: Partners in Time in which the Shroobs were major enemies. This attack also somewhat mimics the Copy Flower item from Partners in Time, although the Copy Flower can theoretically attack forever, while the Magic Window stops after an enemy's health has dropped down to zero.
If a brother is poisoned and use the Magic Window, the brother will, for some reason, lose the poison, but only for the attack.
 Names in Other Languages