This refers to the stage where quantum of dosha increases unto its zenith. Due to which dosha comes out of its ashay (dwelling place) and start circulating through out the body.
In this stage, the toxins accumulated in the GI tract start overflowing.
Generally, up to this stage the damage is entirely reversible and restoration of doshic balance can be achieved with proper measures. Or there may be spontaneous prashama (remission) influenced by seasonal changes. Thus there is sanchaya of Pitta in rainy season, prakopa in fall and prasara in early winter. Based on degree of excitation, it might even passed the stages of prashama or prasara.
Prasara (Leave and spread phase):- It is the third stage of Shat kriya kal in Ayurveda. This third phase of disease formation literaly means to leave and spread. At this phase aama accumulates to such a degree that it finally starts to overflow in the gastrointestinal tract. The doshas transport along with the important nutrients to the dhatus. Its stickiness stops it from being transported back to the gastrointestinal tract. It is the stage of migration.