Some effects can be sustained for multiple rounds. Such an effect is labeled with the word “Sustain.” That word is followed by the name of an action type: “Sustain standard,” “Sustain move,” “Sustain minor,” and the like.
How to Sustain: On the turn that a creature creates a sustainable effect, the creature can do nothing to sustain it; the effect automatically lasts until the end of the creature’s next turn, unless otherwise noted. Starting on that next turn, the creature can begin sustaining the effect.
To sustain the effect, the creature must take the type of action indicated in the effect’s label. For instance, if the effect is labeled “Sustain minor,” the creature must take a minor action to sustain it.
Consequences of Sustaining: When the creature sustains the effect, the effect lasts until the end of the creature’s next turn, unless otherwise noted. Sustaining an effect often has consequences beyond extending the duration. An effect might let the creature attack each time it is sustained, for instance. The description of a sustainable effect notes any such consequences.
Limitations: The creature can sustain a particular effect only once per round and for no more than 5 minutes. During that time, the creature cannot take a short or an extended rest.
Published in Player's Handbook, page(s) 278, Rules Compendium, page(s) 227.