Power Bracelet (パワーブレスレット) is an item from Alex Kidd in Miracle World and Alex Kidd in the Enchanted Castle.
The Power Bracelet allows Alex Kidd to release "Shocking Waves of Destruction" (破断衝撃波 Hadan Shōgekiha, "Breaking Shockwave") from his punches, a powerful ancient technique in the Shellcore art that releases a shockwave from the fist that, like normal Shellcore punches, is able to destroy certain blocks and enemies. This item only lasts for one stage or until Alex loses a life. It doesn't work underwater.
While useful, one should be careful when using the Power Bracelet in some stages as it can destroy blocks that are better not broken, like blocks that allow Alex to reach items or pass by dangerous areas.