It will boost the selected character's Attack, and if timed right, the Defense. It is a Time Level 1 attack. When Geno uses it, red arrows pointing up appear above the character's head. To time it, press any letter button right after the arrows vanish. If the player does it correctly, a thunder effect will occur. If it isn't timed right, "Attack Up!" will appear with no thunder effect. If you timed it right, "Attack Up!" and then "Defense Up!" will appear, including the thunder effect.
When Geno uses this, the attack, or both the attack and the defense will go up to the end of the fight. After the players defeats the enemy, the character's stats go back to normal. Because that attack can be a serious problem for some bosses, like Culex and Smithy, they can use Shredder, which normalizes all of your boosted stats.