Amritsiddhi Yoga forms when a single specified nakshatra falls on a specific weekday (e.g., Monday + Mrigashira, Thursday + Pushya). Classical texts describe the outcomes of work begun during this yoga as imperishable, like the divine nectar (amrit).
Rarer than Sarvarthasiddhi, Amritsiddhi is especially prized for life-milestone beginnings. If it combines with Vishakha or Krittika, however, classical texts flag it as a 'Vish Yoga' to avoid.