The Yoga by which our ancestors have been wise, healthy, prosperous and conquerors of the world since the beginning of creation, this best knowledge of life is like the Philosopher's Stone for us even today as it was in the beginning of creation.

1.  Pranic energy
The body and the universe are a powerful conglomeration of tangible and intangible energies. Physics also believes that matter and energy are the two basic components. In each cell of the body and in each molecule and atom of the universe, 1 lakh to 5 lakh chemical activities take place every moment. When Pranayam is done, these activities increase 10 to 100 times in proportion to the time and the body transforms into a divine energy body. This is what is said in Shrimad Bhagwat Geeta-

Divi Surya Sahasrasya Bhavedyugaduthita.
If Bha: Sadrishi Sa Syadbhasastasya Mahatmanah.
                                                                                            (Bhagavadgita- 11.12)

Therefore, Pranayam awakens and transforms the energy inherent in the body in a positive direction, makes a person healthy and increases his ability infinitely. That is why it is said in the scriptures -
Na Yogat Param Balam- (Gherand Samhita)
On this principle, our revered Gurudev experimented on himself and millions of people of the society. He has freed us from common diseases as well as deadly diseases like Vata-Pitta Kapha Tridosha and cancer, for which Patanjali Institute has scientific proof.
The secret of rejuvenation is- 'Prana'. Wrong diet leads to loss of body power and wrong thoughts lead to loss of mind power. Body and mind become poisonous through anger, worry etc. It is possible to detox them and make your entire existence healthy, happy, peaceful and full of auto-suggestions only through Pranayam.
Basically, the description of Prana Vidya in Vedas is as follows-

Aa Vaat Vaahi Bheshajan Vi Vaat Vaahi Yadrapah.
This is the world disguised as the angel of the gods. 
                                                                             (Rigveda-10.137.3)

Prana is a medicine, meaning oxygen is a medicine. It is not only a medicine, it is a complete medicine. This 'prana' is the carrier of all the divinities of the universe. Prana is a holistic treatment, the basis of prana is complete health. Along with physical changes, the emotional changes that come in the body through prana have very authentic and scientific evidence in our scriptural tradition.
There is a story in Brahmin texts and Puranas that Ashwini Kumars made the decrepit Chyavan Rishi young again. These Ashwini Kumars are none other than Prana-Apan. The advice given by Yamacharya to Nachiketa in Kathopanishad is Prana Vidya. Pranayam is an eternal Divine Therapy.

“Prana evamrita asu: shariram matryam.” 
                                                                     (S.10.1.4.1)

That is, it is only through Prana that we can make ourselves immortal. It is this Vital Force that works day and night without rest from the mother's womb till death. In Brihadaranyaka Upanishad, in the court of Videha Janaka, Vidgadh Shakalya asked Yagyavalkya about the gods, then he described 3003, 33, 6, 2 and 1 god respectively and said that Prana is the main god.
“Katam ako dev iti”? Prana etc.

Sa Brahma tadityachakshate. (Major- 303, 9.9)

Prana itself sits in the centre of everyone (in the heart or womb) in the form of Prajapati and appears in various forms. In Prashnopanishad, Piplad Rishi sings the glory of Prana and says-

Pranasyedam vashe sarvam tridive yatpratisthitam.
Matev putran rakshasva shrischa prajnyam cha vidhehi nah. (Prasnopanishad 2.13)

That is, everything in the three worlds is under the control of Prana. O Prana, protect us like a mother and like a son, give us the boon of wealth and wisdom. The manifestation of such a great Prana Shakti is possible only through the process of Pranayam. Ashtangyoga is like the backbone of Sanatan Dharma. Ashtangyoga has four external parts and three internal parts and Prana acts as a bridge between these two.
Pranayam destroys both our gross and subtle diseases, sorrows and defects and makes us endowed with gross and subtle divine and sacred powers. Maharishi Vyas writes while commenting on Yoga Darshan 2-28 Sutra-

“Yogaanushthanamshuddherviyogakaranam” Yatha Yatha Cha Kshiyate
Inaccuracy: Tatha Tatha kshayakramaanurodhinee gyaanasyaapi deeptirvivardhate.
                                                                                                                                            (Lecture 2.28)

Lingapuraan

Prasad iti samprokta: swante tvih chatushtaye.
                                                                    (Lingapurana-1.8.55.60)

In Linga Puran, the nature of the siddhi (power) generated by Pranayama called Prasad has been described in more detail. It is as follows-

praanopaanah samaanashch udaano vyaan ev ch ||61||
naag: koormastu krakalo devadatto dhananjay:. 
eteshaan ya: prasaadastu marutamiti sansmrt: ||62||
prayaanan kurute tasmaadvayu: praan iti smrt:.
apaanayatyapaanastu aahaaradeen kramen ch. ||63||
vyaano vyaanaamayatyaaghau vyaadhyaadeenaan prakopak:. 
udvejayati marmaani udaanoyan prakeertit: ||64||
saman nayati gatraani samaan: panch vaayav:. 
udgaare naag aakhyaat: koorm milan tu sa: ||65||
krkal: kshutkaayaiv devadatto vijrmbhane. 
dhananjayo mahaaghosh: sarvag: sa mrtepi hi ||66||
iti yo dashavaayoonaan praanaayaamen sidhyati. 
prasaadosy tureey tu om viprashchatushtaye ||67|| 
sarvadvaareshu dehesmin prakaash upajaayate. (geeta-14.11)

Vital Chart