Ayambil Oli is a special Jain practice of nine days of fasting (one plain meal a day, without taste enhancers) done twice a year to honor the Navapad (nine supreme virtues). When people take it as Navapad Oli and keep it regularly for four and half years, it totals 81 days of Ayambil fasting, which is considered a complete, long-term observance of Navapad aradhana. The daily focus is on purifying passions and strengthening right faith, knowledge, and conduct, while meditating on the Navapad.

Key points in simple terms:

• It is observed for nine days, twice a year (during Chaitra and Ashwin).
• If continued for four and half years, it becomes Navapad Oli, totaling 81 days.
• The practice centers on worship of the Navapad and on inner purification through disciplined restraint.

You can read more here and here .