The Power Guitar's normal attack is a single riff which will burst the heads of any zombies within grabbing range. The heavy attack, unlocked after receiving the combo card, is continuous riffing by Chuck/Frank which will immobilize and eventually burst the heads of nearby zombies as long as it is maintained. Beware that the Power Guitar's attack can affect humans as well, this includes survivors. Oddly, it appears to be about as likely to disable a looter or merc as it is to kill a zombie (though it doesn't make their heads explode, it just makes blood spurt out their ears).
NOTE: The Power Guitar is VERY effective against frozen zombies. Sometimes the Power Guitar's waves can go through thin obstacles and still damage zombies.
Dante also used a guitar's music as a weapon, making it so that Chuck, Dante, and Frank (counting Dead Rising: Off the record) a total of three Capcom heroes that have used a guitar as a weapon.