Power Meter

The Power Meter (, K.O. Gauge) is a fighter ability unique to Little Mac in Super Smash Bros. 4 and Super Smash Bros. Ultimate that is shown above his damage meter. It is filled as Little Mac takes or deals damage (even to shields), and each level of charge (represented by the color of the bar) plays a soft, specific sound to notify the player of its charge.

Once the bar is full, a loud bell sound reminiscent of those in a boxing ring plays, and the bar becomes a flashing "KO" sign, causing his neutral special move, the Straight Lunge, to be replaced with a powerful move named the KO Uppercut. While arguably intended to be a one-hit KO per its name, its base knockback is not quite high enough to OHKO at 0%, and reaches KO potential at about 30% when performed on the ground, although this is dependent on the weight of the character; the lightest character in Ultimate,, can get KO'd at 10%, whereas heavyweights like or  do so at about 40%.

When Little Mac is inhaled by Kirby, Kirby copies Straight Lunge, but does not gain a Power Meter and so does not get the KO Uppercut move.

When Little Mac transforms into Giga Mac, the power meter disappears for the entire duration of the Final Smash.

Overview
The Power Meter fills when it reaches 100 units. To visualize this, the meter is separated by ten arrows that represent ten units each. When receiving damage, the Power Meter is filled by units equivalent to how much damage the attack deals. For example, taking 10% will fill one arrow. However, when Little Mac deals damage, the power meter fills with units equivalent to the damage he deals with a ×0.3 multiplier. So if Little Mac deals 10%, he will only fill the Power Meter by three units. As a result, Little Mac will get KO Uppercut much more slowly should he be at an advantage, while he will get it quicker if he is losing, and both methods can stack. Overall, Little Mac is guaranteed to get KO Uppercut if he either deals 333.33...% or takes 100%. In the context of a regular game, Mac will usually get KO Uppercut from around 65%.

Once full, the Power Meter immediately empties once the player presses the special button, whether KO Uppercut is used successfully or not. This means it can be depleted if the player spams the special button during hitstun or other inactionable states. It can also be lost when entering a door in Smash Run. Outside of these scenarios, the Power Meter will be emptied if Little Mac is KO'd, or — after four seconds of maintaining a filled Power Meter — sent tumbling. This means that maintaining the KO Uppercut often necessitates defensive, bait-and-punish gameplay. This is because in the context in which it's obtained, even weak aerial attacks can send Little Mac into tumble, in which he will lose what is likely his only opportunity to use it for that stock.

Origin
The Power Meter appears in the arcade and Super Nintendo installments of the Punch-Out!! series, where successfully landing punches to The Challenger/Little Mac's opponents fills the meter up, while getting hit decreases the meter. Once the meter was full, The Challenger/Little Mac can perform a powerful knockout blow to his opponent. In Mike Tyson's Punch-Out!!/''Punch-Out! featuring Mr. Dream and Punch-Out!!'' for the Wii, the meter was replaced with the Star Punch System. In this release, getting hit causes Little Mac to lose all of the stars he has accumulated, which Smash directly references.

Trivia

 * When the KO Uppercut is successfully used, the camera will briefly zoom in on the move, similar to what happens for a Final Smash.
 * Thanks to the Power Meter filling up when he takes damage, Little Mac can abuse the super armor of his smash attacks to plow through situations that would normally OHKO him or other fighters and fill up his Power Meter immediately: examples include hitting four Blast Boxes at the same time with a smash attack, sending him to high percents but keeping him alive with a full KO Punch ready.
 * The sounds that play on each charge level are so soft that they're impossible to hear under normal circumstances, except for the 10th charge level. If Little Mac is hit with quiet moves like Confusion while the game is in slow motion, the sounds can be heard clearly.
 * There is an unused KO Uppercut animation for Giga Mac. A GIF of it can be seen here.