Shraddha has two main processes – 1. Pindadaana and 2. Brahmin Bhojana. After death, those who reach Pitruloka are invited to the place of Shraddha by mantras. Through the invited Brahmins, they partake in the food. 

The Vedas state that offering food to Brahmins reaches the ancestors:
इममोदनं नि दधे ब्राह्मणेषु विष्टारिणं लोकजितं स्वर्गम्।
(Atharvaveda 4.34.8)
Clarifying this, Manu has written:
यस्यास्येन सदाश्नन्ति हव्यानि त्रिदिवौकसः। कव्यानि चैव पितरः किं भूतमधिकं ततः॥
(Manusmriti 1.95)
'Through the mouth of Brahmins, the gods consume Havis, and the ancestors consume Kavis.'

The ancestors are not visible because their bodies are sukshma (subtle). Manusmriti mentions that the ancestors reside secretly within the invited Brahmins during Shraddha. Like prana (life force), they move when the Brahmins move and sit when they sit. During Shraddha, they accompany the Brahmins, partake of the food, and become satisfied.

The Shatapatha Brahmana also says:
तिर इव वै पितरो मनुष्येभ्यः।
(2.3.4.21)
'The ancestors are hidden from humans due to their subtle bodies.'

That’s why offering food to Brahmins in Shraddha is very important. The ancestors are directly present within their bodies at the time of Shraadha.

Knowledge Bank

What does Ugrasrava mean?

A person who has great hearing is known as Ugrasrava. Hearing means grasping power. He was a primary narrator of the Puranas.

Drona meaning

Droni means cave in Sanskrit. The sperm of Sage Bharadwaja grew up into a human being in a cave. That's how Drona got his name. Drona- someone who was born and grew up in a droni, cave. This is as per Bhava Deepa commetary of Mahabharata - Adi Parva - 63.106.