From the Super Mario Wiki
Stealth Burst is the Mii Gunner's second side special move option in Super Smash Bros. for Nintendo 3DS / Wii U. Similar to Zelda's Din's Fire, the Mii Gunner launches an invisible projectile that explodes a short distance away; the distance the projectile travels and the power of the explosion can be increased by holding down the special attack button, and will explode when the button is released. The player cannot control the direction the projectile travels, and it will automatically explode upon hitting a surface. If used in midair, the Mii Gunner will be rendered helpless when the projectile explodes.