Fasting during Paryushan Parva is a central way Jains practice self-control, purification, and forgiveness. Here’s a simple overview.

• Why fast: to reduce karmic bondage, strengthen self-discipline, and turn focus to spiritual practices like prayer, reading, and reflection. It often goes hand in hand with Samvatsari (forgiveness) and daily prayers.
• Common fasting types (upvaas):

- Upvaas: full-day fast with no solid food; typically only boiled water, and only during daylight hours. - Ekasana: eat one meal a day, usually before sunset; water may be consumed between sunrise and sunset. - Beasana (Biyasan): eat two meals a day, still with restrictions. - Ayambil: one bland, tasteless meal per day, without spices, milk, ghee, oil, fruits, or vegetables. - Attham: fast for eight days (Paryushan), with boiling water allowed during the day. - Varsitap: a more intensive pattern often alternating days of fasting and a small meal.

• Dietary rules during fasting (typical for Paryushan):

- No eating after sunset. - Boiled water is commonly allowed, usually after sunrise and before sunset. - Avoid root vegetables (onion, garlic, potatoes, etc.), fermented foods, and strongly-spiced items.

• Additional practices during the period:

- Samayik (meditation for a set time, often 48 minutes or more). - Pratikraman (confession and repentance), especially on the last day of the festival. - Chaitya Paripati (temple visits) and Kshamapana (asking forgiveness from others).

• Last day significance:

- Saṃvatsarī (the final day) often includes a long Pratikraman and a formal confession, followed by forgiveness rituals (Micchāmi Dukkaḍaṃ).

Notes on tradition differences:

• Digambar and Śvetāmbara traditions emphasize the same spirit of restraint and purification, but the exact duration and emphasis of fasts can differ (e.g., some communities emphasize the eight-day Attham/Atthai fasts or the eight-day Saṃvatsarī observance somewhat differently). The core aim—self-purification and forgiving conduct—remains the same.

You can read more here and here .