Sakshi means observer or witness who can stand up to an audience and tell authoritatively what happened factually. Sadhana is a karma which bears fruit. The fruit of it is the mantra energy which is accumulated in the firm merits. These are offered to the concerned deity and in return of it desired boon is taken. If there are no sakshis present then there's no proof of your performed karma hence the energies can cheat the sadhak while giving boon. Hence these pancha sakshis are established and worshipped essentially in pre preliminary puja procedure before starting any sadhana, who inturn are witnesses for our sankalpa, karma and siddhi. They also protect is and our sadhana from evil influences.
Who are the pancha sakshis?
2) Kalasha (aapa/Varuna)
3) Gau (Kaamdhenu housing 33 koti devatas)
4) Guru (Shiva/ishta)
5) Surya/ Chandra (Sarva sakshi)
Apart from these some people also consider Ganapati and bhairav as sakshis too.
1) Deepak or lamp is lit as it is the agni very purohit of devatas, carrier of havya to all the parts of multiverse to devatas as well as asuras. Who can cross question it's witnesses? Over that it removes darkness and ignorance from our minds and surroundings. It also destroys many vighnas caused by negative energies in the environment. Therefore it's the primary sakshi without which no sanatani puja, worship or sadhana is done.
2) Kalasha: Water is the very source of life, it's the great and mighty Varun the god of judgement. Again it's the carrier of offerings to the devatas and other yonis. Kalasha holds bramha at it's base, rudra in it's throat and vishnu in it's mouth. It has matrukas, 33 koti devatas in it. It is again sakshi of all auspicious karmas.
3) Gau/ cow: Cow as we know is the representation of purity in Hindi dharma. Serving, praying to a cow earns very high merits. It again houses 33 koti devatas. Worshipping cow and doing go daan gets us huge punya. Hence it's worshipped as a sakshi before sadhana to improve our chances of success.
4) Guru: Guru is the manifestation of supreme consciousness. It is the combined form of bramha, vishnu, Mahesh. He is the parabramha. He's none but one's ishta devata. Just having the guru with you in sadhana increases the chances of success by 100%. Over that Guru's grace and presence makes sure that the invoked energy gives the fruit of the sadhana truthfully. If nothing is there then only guru pujan is said to be enough before starting the sadhana.
5) Surya/Chandra: These are the universal witnesses of all the events always present in the sky. They are known to be unbiased about every being. They give us the light to end our ignorance and power of truth to achieve our goal. They give us dominion over our physical body and mind. They're worshipped during Sandhya Vandan before starting any puja or by establishment of Surya and Soma mandal.
Ganapati and Bhairav are worshipped primarily for removal of obstacles from the karya and protection from perils caused by negative entities, enemies and outsiders. They can also be the sakshi of one's sadhana adding more weightage.
So do the pre preliminary puja procedure thoroughly before starting any mantrik, yantrik or tantrik sadhana.
*Note : These things do not apply that stringently on hatha yoga or meditation sadhanas.
🙏 Namo Narasimha 🙏.