Wikipedia
Prānapada is a necessary special lagna calculated from the position of the Sun at sunrise; the Sun is the giver of life and Prānapada indicates the life energy.
The Aitareya Aranyaka reminds us that Prana (breath) is sound, for every breath when it sounds, sounds loud, as it were. Prāna is Sukta ('well-said'), Prāna is the Rik ('verse'), Prāna is the Pada ('word') (Aitareya-āranyaka II.ii.8).
Prānapada is a certain sensitive point which can be obtained by adding twice the birth-time (in vighatis) to (a) the Sun’s longitude, (b) the Sun’s longitude + 240 degrees or (c) the Sun’s longitude + 120 degrees according as the Sun is in a moveable, fixed or common sign; the duration of one Prānapada is 15 vighatis or 24 seconds or 2 degrees-arc, the Prānapada sign can be found by deducting the multiples of 12 signs or rasis from the figure thus arrived.
The birth will be auspicious if Prānapada falls in the 2nd (wealthy, fortunate), 4th (good-looking, sincere, faithful, truthful), 5th (doer of good deeds, adept, happy), 9th (intelligent, wealthy, fortunate, serves others), 10th (powerful, intelligent, skilful, expert in carrying out royal orders, pious) or 11th (famous, wealthy, fair-complexioned, attached to mother) bhava counted from the lagna, in other bhavas it indicates an inauspicious birth.
The concept of Prānapada lagna appears to be based on the law of periodicity according to which humans, animals and plants could occur only at certain definite intervals. Parasara suggests that in the matter of birth-time rectification Prānapada, Chandra and Gulika have all to be considered.