Jet Hammer is King Dedede's Down Special Move. If you hold down the Control Stick and the special-move button simultaneously, you can charge up his hammer. You can even walk around and jump with it charged up, but you 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 you activate the attack. However, once it your damage reaches 150% and over, you will be no longer be damaged by the minor damage. Fully charged, the hammer does massive damage and knockback. Although his forward smash does far more knockback, range, and possibly damage, this attack allows you to move, making it easier to edgeguard with it, or hit a falling enemy. It also 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 tiling 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.
A little known fact is that an almost fully charged Jet Hammer (does 30% damage) has greater knockback than a fully charged one (which does 31%).
Origin
Kirby is seen performing a similar move with the hammer in Kirby Super Star, Kirby and the Amazing Mirror, and Kirby: Squeak Squad. The jet engine inside the hammer is a new feature shown in Brawl.