While the Raga has some specific rules you have to follow so that the process of creation and expansion within that structure will give rise to the proper emotions and moods, exceptions exist in that these rules are allowed to be broken by those who can handle the improvisations expertly and cleverly, as in the case of a pilot, who after knowing the complex technique of handling a flying machine, is able to display an unlimited number of aerial feats which would be difficult for a beginner to perform.
In an effort to create and reinforce the Raga image the expert performer, or composer adds certain shades of other Raga melodies with great skill, taking it to the limit, expertly maneuvering in ascending and descending movements, until finally bringing the melodic shape of the Raga back to its original form. This process of camouflaging a Raga temporarily is termed Tirobhav or deviation. Coming back to the original Raga form after the deviation is called Avirbhav.
This state of change however should not be maintained for a long period, as this would mean over-indulgence in a different tonal field. Instead, the player immediately and expertly comes back to the tonal grouping of the original melody.