From SmashWiki, the Super Smash Bros. wiki
The Hammer head is an item in Super Smash Bros. Melee and Brawl. It only appears when it falls off a Hammer; it can be thrown for powerful damage. However, in Brawl a Golden hammerhead will never fall off. The equivalent of a headless hammer on a Golden Hammer is the Golden Squeaky Hammer. In Melee, the bonus Headless Hammer is awarded if the Hammer Head falls off the player's Hammer.
[edit] Using the Hammer head
If you get your hands on one, be sure to Smash Throw it for maximum effect. Whenever you throw it, rush up and catch it, because it will disappear if it touches the ground. This does 35% on a Forward Smash Throw, and up to 37% on a Up
Smash Throw, and it has HUGE knockback, so use it wisely.
If your opponent has a Hammer Head, be sure to reflect it if you can. If you can't, then be ready to block it. If you succeed in doing so, press A after the Hammer head bounces off your shield to catch it.
