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 untill you actvate the attack. 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.
Origin
Kirby is seen performing a similar move with the hammer in Kirby Super Star and in Kirby: Sqeak Squad. The jet engine inside the hammer is a new feature shown in Brawl.
Role in SSE (Adventure Mode)
King Dedede used this to turn Luigi into a trophy, which he used to bait Wario so he would exit his vehicle, after, when he was picking up the Luigi trophy, the Waddle Dee Army surprised him, and forced him to throw the trophy into the vehicle, which King Dedede was entering, which after, leaving with all three trophies (The princess you didn't save, Ness, and Luigi) and the Waddle Dee army following him, leaving Wario behind.