Chaining is a game feature that is only available in Kingdom Hearts 358/2 Days. Chaining increases the amount of Heart Points collected. While the initial amount of increase is diminutive, chaining is often necessary to gather enough Heart Points to complete a mission fully, with all rewards. The increase becomes higher if the chain continues by defeating more Emblem Heartless.
To start a proper chain, an Emblem Heartless must be defeated. It must be done in the limited time before the chaining time runs out. The time left to continue a chain is shown by a yellow circle around all enemies in sight. The circle will gradually become smaller as the chaining time starts to run out. Abilities like Chain Time can increase the time limit. Chaining a boss Emblem Heartless will give a significant increase. However, if you defeat a Pureblood Heartless and then defeat an Emblem Heartless such as defeating a Shadow and then a Yellow Opera, there will be no increase. However, Pureblood Heartless can hold the chain and keep it going. For example, defeating a Yellow Opera, Shadow, and Yellow Opera will cause a chain, though the Pureblood Heartless do not produce any Heart Points themselves. Maxing out a chain is also special. If an Emblem Heartless is defeated when the chain is maxed out, the increase will be the equivalent of the number of hearts the Heartless releases normally.