Also when two characters Impulse gauge are full they can perform a double Impulse where they team up to perform better techniques. They can also be done with three people and four to make a numerous amount of different techniques.
Impulse can be gained by many different ways e.g. The number of times a character is hit, the number of times a characters attacks. Also things like healing someone can increase the gauge for healers to use and the amount of magic and so on.
Gaining, Max, Ultra and Master Impulses is different though. On your journey you will meet certain mini quests, which will result in you getting one, although some are on par with the storyline. Obtaining the Impulse doesn’t affect the storyline in any way but small secrets about character’s pasts may be revealed upon finding one.
Different characters gain Impulse in different ways though. See Character techniques for more details