The move has a similar usage to Kirby's Hammer from the previous two games, being a slow yet strong blow that hits opponents at medium range. However, it can now be charged with functionality like Jet Hammer, allowing Kirby to move around and jump while charging the move. Also, like Jet Hammer, Kirby can charge the move indefinitely, but will receive recoil damage for as long as the move is not used after being fully charged, though this damage is not inflicted if Kirby's damage reaches or is above 100%. Unlike in Brawl, the hammer doesn't stall Kirby's momentum in the air, and therefore can no longer be used for recovery. An uncharged swing is significantly weaker than a swing in Brawl, and the aerial version has slower startup. The move now uses a flame effect, though not when uncharged.
When fully charged, Kirby will bring the hammer further behind himself with brighter flames, with Kirby flashing a reddish-orange color. The animation, instead of the usual sideways swing, is instead an upwards one for both the ground and midair versions. The move is able to KO Mario as early as 25% damage on the center of an Ω form stage, and can KO at an average of 40%. It also has super armor from frames 11-13, as well as intangibility from frames 2-10. Unsurprisingly, this makes Hammer Flip, knockback-wise, Kirby's most powerful attack. It has less knockback when used in the air, but Kirby still swings twice if the attack is not fully charged.
In the NTSC version of Smash 4, the move is called Hammer Flip, the same name as a different move usable by Hammer Kirby in the Kirby series that is his Up+B attack in the games. The move is still called Hammer in PAL regions, and the name "Hammer Flip" refers only to the fully charged version of the move. In Ultimate, it is called Hammer Flip regardless of the console's regional settings.
Special Move customization was added in Super Smash Bros. 4. These are the variations:
Use by others
Hammer is one of Kirby's many Copy Abilities from his various adventures, most frequently obtained by inhaling the mini-boss Bonkers. It is often one of the most powerful ones, dealing massive damage to enemies. The hammer is almost identical in appearance to King Dedede's. In Super Smash Bros. 4, the move was altered from its previous incarnation to be nearly identical to the ability's Hammer Flip move, including its ability to catch on fire. In Kirby Super Star and Kirby Super Star Ultra, the aforementioned Bonkers, as a helper, is also able to perform his own version of the Hammer Flip due to sharing Hammer Kirby's moveset (but he brings his hammer down instead of swinging upwards, unlike his appearance in Smash Run).
The Giant Hammer custom variation is most likely derived off of one of the Hammer ability's moves only in Kirby: Squeak Squad, where Kirby can enlarge his hammer for a massive delayed smash (but the attack must be learned through its Ability Scroll first).
In Kirby: Planet Robobot, Smash Bros. Kirby obtains the ability to charge his regular hammer strike into a fiery Hammer Flip, and also gains the ability to move around and jump while charging, referencing Kirby's updated moveset in Super Smash Bros. 4. These changes carry over to the regular Hammer moveset in Kirby Star Allies, though it no longer catches on fire unless it is imbued with the Sizzle element.