By pressing the special move button, Donkey Kong will start swinging his arm around, automatically charging a wind-up punch that, when thrown, causes great damage and knockback if it hits, especially when fully or nearly-fully charged.
For every time that Donkey Kong swings his fist, the charge advances to its next stage, entailing an increase in the damage of the punch. After any one of these swings, the charge can be cancelled and stored to be continued later: by pressing the shield button while charging, Donkey Kong will shield, and moving the control stick left or right will cause him to roll. If Donkey Kong suffers knockback while charging, he will invariably lose all of his charge.
At any time while Donkey Kong is swinging his fist around, the button can be pressed again to deliver the blow. If the button is not pressed, then after ten swings, Donkey Kong will drop his fist and his head will begin to smoke, indicating the punch is fully charged. At this point the button only needs to be pressed once to administer the attack. From Brawl onwards, a fully charged Giant Punch grants Donkey Kong armor when used, allowing him to power through an enemy's KO attack while he uses one of his own in the process. However, a Giant Punch that has just one stage below full charge has noticeably higher knockback.
From Melee through Smash 4, the move becomes markedly weaker if used in midair, and also causes Donkey Kong to become helpless, making it very risky and much less rewarding to use in aerial combat compared to its utility on the ground.
Aesthetically, the move has been through a few changes. In Smash 64, Donkey Kong has his right hand directly behind him while his fist winds up. From Melee through Smash 4, Donkey Kong uses his left arm for the attack, while also winding up his arm beside himself. The move was the slowest in Smash 64 with some notable recovery, though the animation for both the actual attack's startup and its recovery has been sped up in later games. In Ultimate, Donkey Kong now charges Giant Punch with either arm, due to his mirrored stance. It also no longer puts him into a helpless state, similar to Smash 64. After fully charging his Giant Punch, Donkey Kong now displays an angry expression until he unleashes his Giant Punch, though he shows a regular expression by canceling Giant Punch with a shield button just as it is about to get fully charged.
As explained above, Giant Punch's charge goes through stages. These are visually recorded in the swings of Donkey Kong's arm; each time he swings, the punch's damage and knockback is generally increased. The charge has eleven stages in total; the eleventh stage is the fully charged stage. A fully charged Giant Punch will have a quicker animation, resulting in a quicker punch, and do the most damage, but is weaker in knockback than a few of the stages below it. From Melee onwards, all stages of the punch are weaker when used in mid-air.
Smash 64 stages
Ultimate abandoned the concept of storing Giant Punch in stages, instead opting for a charge formula in line with other moves. In this case, Giant Punch's power will increase by a given amount every 6.67 Frames. The charge duration lasts 110 Frames as of 6.0.0, and overall it will increase in power by 18%. This means it will deal 28% maximum. As usual, an uncharged Giant Punch is more powerful than a maximum charged one. However, to utilize this takes extreme precision in Ultimate due to its frame perfect nature and no longer charging via windups.
Charging from nothing to full takes 121 frames, including the 7 frame pre-charge lag and 4 frame post-charge lag (which can also be applied to shielding, rolling or jumping). It can also be cancelled through jumping in the air, which takes 5 frames as well.
Ultimate introduced a glitch that caused the Z-Axis to make Giant Punch miss point-blank opponents. This was fixed in 3.0.0, although only for the fully charged version. 3.1.0 fixed a glitch with Giant Punch that existed since Smash 4, where landing on frames 12-17 during a fully charged Giant Punch caused DK to never get his super armor.
Reverse Charge Cancelling
Reverse Charge Cancelling is the concept of reversing Giant Punch's windups, and has existed since 64, and stayed in every Smash game to date. Normally it is impossible to B-reverse the windups, but through inputting a normal turnaround B, this can occur. This provides some additional unique maneuvers for Donkey Kong, especially in later games where charge cancelling becomes more prominent.
Since Melee added the ability to cancel Giant Punch in the air, it became possible for Donkey Kong to use Reverse Charge Cancelling for movement. Through this, he could Reverse Charge Cancel, cancel the Giant Punch windups, and follow up with an aerial. This added many of the options high-level players still use today, such as the ledge options it provided. RCC back aerial was a powerful way to avoid ledgetraps. The ability to use reverse aerial rushes through this added powerful mixup options as well, adding further depth to Donkey Kong's playstyle. It's also possible to chain these together, possibly allowing for a powerful punish if the opponent tries to jump in. This gives Donkey Kong better bait and punish tactics that would otherwise be very difficult to do.
In Ultimate, the ability to jump cancel charge moves greatly improved Reverse Charge Cancelling. This made Reverse Charge Cancelling much more accessible to players, not having to simply cancel Giant Punch and then jump. This greatly streamlines Donkey Kong's playstyle. It also provides Donkey Kong with an extra movement option, as the momentum allows for him to weave in and out of the opponent's range, especially with the ability to jump now readily available and not requiring more frames to cancel Giant Punch.
Special Move customization was added in Super Smash Bros. 4. These are the variations:
While the move has no true origin from his home series, Donkey Kong has done a similar move to this one in other forms of media outside of Smash Bros.: