If the player holds down the control stick and the special-move button simultaneously, they can charge up his hammer. The player can even walk around and jump while the hammer is being charged, but they can't save the charge and cannot cancel the move, unlike most other charging moves. Once it reaches full charge, the player can hold the charge indefinitely, but it will cause one percent of damage to King Dedede every second until the player releases the attack. However, once Dedede's damage is high enough, damage from holding a fully charged Jet Hammer will no longer occur.
Fully charged, the hammer deals massive damage and knockback, and the last few stages before fully charged deal even more knockback. However, despite its extreme power potential, Jet Hammer is considered Dedede's worst move in Brawl and is barely ever used in competitive play, with it having almost no practical use. Jet Hammer charges up slowly, and requires considerable amounts of charging before it reaches power levels beyond Dedede's other KO moves (which can all be completed before Jet Hammer charges enough to reach their power). While Dedede can move and jump around while charging Jet Hammer, he moves and jumps too slowly for it to make it useful, and any opponent can easily outmaneuver a Dedede charging Jet Hammer. Additionally, Jet Hammer has poor reach and surprisingly small hitboxes, which when combined with the aforementioned very slow movement and long charging time, makes it incredibly difficult to land the move on an opponent that isn't incapacitated. To make matters worse, it has 18 frame startup just on the charge, and a Dedede player has to wait 11 frames for the hitboxes to actually come out (although at full charge it comes out 3 frames sooner on Frame 9). Since it has high ending lag (FAF of 60 on release if not fully charged, 70 if it is) and cannot be cancelled, combined with the aforementioned movement and reach issues, along with it having transcendent priority and no armor whatsoever, Dedede is completely defenseless when using the move. This gives opponents a virtually free punish if Dedede tries to use the move while they're not incapacitated.
The final nail in the coffin is that while Jet Hammer can be incredibly powerful, fully charged it's still not as powerful as Dedede's uncharged forward smash and pales in comparison to a charged one. Additionally, while Dedede's forward smash has extremely long start-up lag, it'll still come out well before Jet Hammer reaches anywhere near fully charged, and unlike Jet Hammer, it has massive reach and rather low ending lag, along with a sourspot that extends its reach farther. This makes the move much less punishable and actually easier to land. As such, whether it be capitalizing on an incapacitated opponent or trying to go for an especially early KO, Jet Hammer is obsolete to Dedede's forward smash, and the other mentioned issues keep it from being any sort of weaker but more reliable KO move. Unless the player wants to mock an opponent whose shield is broken, there is never any real excuse to ever use Jet Hammer in high level play.
There is a glitch that increases the effectiveness of this move by allowing King Dedede to skid across the stage at incredible speeds while charging it. This is done tilting the control stick in either left or right direction just barely enough to allow Dedede to move. If done properly, Dedede will continuously take only the first step in his walking animation and move across the ground at an incredible speed, which can be seen here. However, the extreme precision this requires to maintain beyond a split second makes it beyond the technical ability of human players, and leaves it as something only seen in tool-assisted demonstrations.
Unlike other chargeable moves like Eruption, Jet Hammer's damage does not increase in stages, but rather in a continuous fashion which includes decimal percentages. As such, the below table states general kill percents at different levels of charge in Brawl, and is not an exact guide. Like most chargeable moves, the fully charged state is not the most powerful; however, it does deal the most damage.
Jet Hammer returns as King Dedede's down special. It has been heavily buffed from Brawl, where it now has much greater range, a much larger hitbox, and the full charge is more damaging on the ground, dealing up to 38% damage, and can KO below 50% when near stage edges (though it still deals less knockback overall than the weaker charge levels, due to having only 5 BKB and 66 KBG). The full charge also deals high shield damage, despite lacking a shield damage bonus. Additionally, overcharging the move now stops damaging Dedede at 100% instead of 150%, and an overcharged strike is capable of hitting any ledge-hanging opponent, giving it a new use as an edgeguard, especially due to SSB4's new ledge mechanics.
Visually, the lighting emanating from it is much more intense, noticeably lighting up the surrounding areas, and now has sparks ejecting from the hammer. It also produces wind visuals. When charging the move, Dedede now has a grimacing facial expression and can be seen visibly sweating.
Despite Jet Hammer's buffs in the previous installment, the move has been further improved. Jet Hammer now deals higher damage both in the air (10%-27% → 11%-32%) and on the ground.(11%-38% → 12%-40%). Its shield damage is now strong enough to break full shields when fully charged. The hitboxes near the handle now do equal damage as the hammer's head, removing the sourspot. Finally, Jet Hammer has 14% damage-based armor during the swing when used on the ground, from frames 1 to 14. However, the armor can be used in the air through using windboxes to push Dedede offstage. This makes it a somewhat useful, albeit committal option on the ledge. While the move is still considered situational, it has been improved to become a much more devastating attack, one that can take stocks if the opponent does not react properly.
Aesthetically, the fire from Jet Hammer is changed from blue instead of red, and a fully charged Jet Hammer triggers Special Zoom upon hitting an opponent.
Special Move customization was added in Super Smash Bros. 4. These are the variations:
In some games, starting with Kirby Super Star, Kirby can perform a similar move, the Hammer Flip, when in possession of the Hammer copy ability: it consists in charging the hammer, then releasing it with a fiery upward swing. Later games, released after Brawl, made the Hammer Flip available to King Dedede himself in his playable appearances. The jet engine inside the hammer is a Smash Bros. exclusive feature; however, King Dedede's Masked Dedede persona in Kirby Super Star Ultra (released eight months after Brawl) wields a new, electrified mechanical mallet which can shoot missiles and spew fire.