MP Charge is an automatic ability appearing in Kingdom Hearts II and Kingdom Hearts II Final Mix that occurs when the MP bar is reduced to zero. The blue bar is swapped out for a pink one, which steadily decreases until it reaches zero. During this time, magic may not be used. When the pink bar reaches zero, the blue bar returns, allowing use of magic until it reaches zero again.
Using Ethers and Mega-Ethers will decrease the bar. Using an Elixir or a Megalixir will decrease the bar completely, along with restoring your health.
Equipping MP Haste will make the bar deplete faster, depending if MP Haste, MP Hastera, or MP Hastega is equipped. Equipping MP Rage will not affect the MP bar while it is restoring however it does work while you have magic and it restores a certain amount of MP depending on the damage taken.