Jet Hammer (ジェットハンマー, Jetto Hanmā) is King Dedede's down special move. 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 with the shield button. If it is charged up too much, it will cause one percent of damage to King Dedede every second until the player activates the attack. However, once the damage reaches 150% and over, the player will be no longer be damaged by the minor damage. Fully charged, the hammer does massive damage and knockback. Although his forward smash has more knockback, range, and possibly damage, this attack allows the player to move, making it easier to edgeguard with it, or hit a falling enemy. In addition, it has much less windup lag, although, in return, like Quick Draw, it is easily telegraphed.
There is a glitch that increases the effectiveness of this move by allowing King Dedede to skid across the stage at incredible speeds while retaining a full charge. This is done by charging the hammer until Dedede begins damaging himself, and then tilting the control stick in the direction you want to move 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.
In Brawl, Jet Hammer has stages of damage and knockback that are decided by the charge, there are ninteen stages of charge on the ground and another ninteen when in midair. Stages 1-18 are the charging stages while stage 19 is the fully charged stage. Like most chargable moves, the fully charged state is not the most powerful.
 Description from the Instruction Booklet
Charge up with B while walking around. Charge for too long and you'll take damage.
Kirby is seen performing a similar move with the hammer in Kirby Super Star, Kirby & the Amazing Mirror, and Kirby: Squeak Squad called the Hammer Flip (which was possibly integrated into Kibry's side special). The jet engine inside the hammer is a new feature shown in Super Smash Bros Brawl. In a later Kirby installment, Kirby Super Star Ultra, King Dedede made an appearance with a new mechanical mallet, however its appearance is much more exaggerated. It had the ability to shoot missiles and spew fire, as well as being electrified. Since Kirby Super Star Ultra was released 6 months after Super Smash Bros. Brawl, the mechanical hammer was a Brawl-specific creation. However, it is possible that this was inspired by Brawl.