What is Nisargayu (निसर्गायु)?
This method cannot be applied to all cases. According to Kalyana Varma (Saravali) and Manitha, Nisargayu can be applied to those horoscopes in which Moon is the strongest.
Nisargayu method is exactly like Pindayu method. The only difference is that the planets give different years at their highest exaltation degrees.
Venus 20 yrs. Saturn 50 yrs This method cannot be applied to Indira Gandhi's case because Moon is not the strongest planet among the Lagna - Sun-and Moon. If it has to be worked out as and exercise or for examination purpose then the above mentioned method should be used.