There was an interesting story behind using Vishnupada during Shraddakarya. Gayasura was a great devotee of Lord Vishnu. He worshipped Vishnu for a long time. Lord Vishnu appeared before him and asked to take the required boon. Gayasura asked to make his body so pious that every one who sees him will attain Mukti (salvation) immediately! Vishnu accepted it and disappeared.
The Actual story: Gayasura started to grow his body to an enormous size with the help of his yoga shakti. It was hundreds of kms in height. His wish was to show himself to all so that everybody on the earth attains Mukti. This made Indra (the king og deities) fearful. If everybody attains Mukti easily then what about the sins committed and the Law of Karma? He prayed Trimurthis, Brahma, Vishnu and Maheshwara. He asked them to tell a solution. They laughed at Indra’s request and disappeared.
They approached Gayasura in the form of three Brahmins. Gayasura welcomed them. They first requested him to stop growing as it was against the Law of Karma but Gayasura would not listen. Then they told Gayasura that they are going to do a very sacred Yagna and for that requested land from him. Gayasura happily accepted to give it. But they questioned him where will he give the land? Each and every place on earth has bad incidents in it’s history and so unfit for their sacred Yagna.
Gayasura wondered what he should do. He asked them to tell the solution. They told that his body will be the best place! They supported their logic with the boon he got from Vishnu and his wish in giving salvation to everybody! And told that this can be considered as a pious challenge to him and that after the Yagna was over, he can continue to grow in height and give salvation freely to all! He has to face many Vighnas (obstacles) both physically and mentally to fulfill his desire. They told that the capabilities of Gayasura can be pre-tested in helping this Yagna.
Gayasura accepted. That Yagna was conducted on the pious body for seven days. He developed his etheric body with his power and slept on the earth. His head was in Gaya of Bihar (Siro gaya), his navel was in Jaipur of Orissa (Nabhi gaya) and his foot were in Pithapuram of Andhra Pradesh state (Pada gaya). The Brahmins started doing Yagna (Legends say his body was cut and thrown in 3 pieces but then that is not correct as he did not do anything wrong to be rendered that punishment only that in his over-enthusiasm and asuric nature to exaggerate anything he tried to obviate the ignore the Law of Karma).
Gayasura bore the fire on his body. He was counting the days with the help of crowing of cocks in the early morning. He became frustrated that day by day because of the fire but not moving his body for the word he gave. On the final day Lord Shiva took a form of the cock and crowed at midnight. Considering it as the sign of morning Gayasura woke up. The Yagna was destroyed. The Brahmins felt very sad. They asked him that if he could not control himself for just seven days how he is willing to bear the sins of entire humanity for unlimited time by growing in size? Gayasura realized his mistake. He agreed his fault. However, Trimurthis appeared before him and told that his body will be present as it is and the entire land will become sacred because of him only. He was granted Moksha too and blessed. He entered into Samadhi and nirantara tapascharyam and stopped his asinine act of growing in size.
Lord Vishnu put His one lotus foot down on Gayasura’s body and pressed His lotus foot down so hard the Gayademon gave up his body. Thus he also went back to Godhead, thus there still is the lotus impression of the Lord Foot and everyday this Lotus foot of the Lord is worshipped. Foot of lord Vishnu is also known as “Vishnupada” placing the PINDA on it will facilitate our PITRUS to get a purest form of our contribution and it was one of the wish of Gayasura also.