From the Super Mario Wiki
The Power Throw is a move Wario can do in his various games. Regular throws are sometimes too weak to defeat enemies (bosses included), but Power Throws can defeat larger and stronger ones. If the player holds the button, Wario will charge up and start to flash a white-blue color (in Wario Land II, this does not happen). Upon releasing the button, Wario throws the enemy with incredible power; the enemy is usually destroyed when it hits the nearest surface.