From the Super Mario Wiki
Counter Surge is Corrin's down special move in Super Smash Bros. for Nintendo 3DS / Wii U. Similar to the Counter used by other Fire Emblem characters, Corrin braces himself for an attack, and when struck, fully transforms into dragon form and stomps the ground, counterattacking with jets of water that strike opponents from both sides, and launching them vertically.
Corrin's counterattack deals 1.2x the damage of the blocked attack, with a minimum of 10% damage. In update version 1.1.4, the counterattack multiplier was originally 1.3x before being weakened in version 1.1.5.