In Kingdom Hearts, once Sora is given his first magic spell from Donald in Traverse Town, he gains the ability to cast magic from the Magic or Shortcut menus. All magic, techniques, and summons consume a certain number of MP (with each segment in the MP Bar representing one point), and when the MP bar has been completely exhausted, no more spells can be cast.
However, as Sora uses physical attacks or, with a certain ability equipped, takes damage, a small orange bar along the outer rim of the MP bar rises. The extra energy exerted by attacks is absorbed and turned into a subunit of MP known as Charge, represented by the aforementioned orange bar. Each time the Charge Gauge reaches a certain point beyond Sora's current MP, it is converted into one new point of MP, which is automatically added to Sora's MP Bar, where the Charge Gauge will start over from the bottom of the MP Bar. Some spells (like Fire) consume Charge before they consume MP, making them less costly to use.
Some players, often those who focus highly on physical attacks, will depend solely on the use of the Charge Gauge to gain back lost MP, leaving Item slots open for Potions and other HP restoring items. However, it should be mentioned that the more MP Sora has, the longer it takes the Charge Gauge to convert into one point of MP, since the gauge must reach one point higher than the current MP total.
Charge can be gained by picking up the clear MP Balls dropped as prizes from enemies, attacking with physical attacks, or by sustaining damage while the "MP Rage" Ability is equipped (costs 2 AP). The more damage that is dealt (or taken), the more Charge will be gained.
Some fans of the series were disappointed at the loss of the Charge Gauge for the inclusion of Kingdom Hearts II's double-edged, MP Recharge system. Although sharing the similarities of the Charge Gauge, the MP Recharge of the series' second installment is different in execution. When all of Sora's MP is consumed, the MP Bar converts from blue to purple and slowly refills. While MP Recharge is activated, no spells can be used.
One difference that could be seen as beneficial to the MP Recharge is that any spell, regardless of how much MP it costs, can be used, even if there is insufficient MP to do so, much like an overdraw. Although, the player can select what spells they would allow to overdraw and activate the MP Recharge, and what spells may not. Another benefit of the MP Recharge is that by equipping the "Berserk Charge" ability, Sora can perform an endless combo, although, this too, has it's drawback, since to defeat bosses you either need to deal the finisher of a combo or hit them with magic.