In plain words
Bali Cakravarti, a rākṣasa, may take the whole universe into his hands and turn the whole land into a land of rākṣasas. So protect us, and You must resolve on a way to keep him from his aim," they prayed. Men are narrow of heart. They are not able to recognise the truth that while man thinks one thing, God thinks another. Nārāyaṇa agreed outwardly, looked into Bali Cakravarti's pure heart, was sure that he was a great devotee, and made the holy resolve to send him to heaven one way or another.
In keeping with this resolve, He was born as Vāmana in the Siddhāśrama. Vāmana entered the sacrifice seeking a gift. Because Bali Cakravarti was pure of heart and selfless, he could see Vāmana's radiance clearly and recognise Him as God. "Swāmi! At the time of this sacrifice, what is Your wish? I have resolved to give up every tie, to give in charity to everyone and make my life worthwhile," he said. Vāmana was very small. The bird is small; its cry is mighty. The wish He asked was very small, but in that very wish the endless universe lay contained. "Oh no! Bali Cakravarti, I do not need a great gift. Give Me just three feet of ground and that is enough," He said.
"For one who holds sway over the whole cosmos, is giving three feet a matter of moment?"
But this Vāmana took on the Avatāra of Trivikrama. He took in the whole world in one stride. He trod down the sky with another stride. "Bali Cakravarti! Can you show Me any room for the third stride?" He said.
"The very heart You gave me, O Lord, that same heart I offer You in turn, O Glorious One; what else could I bring for the worship of Your feet?"
"Here is the head; set the third foot right on it," he said. That is to say, Bali Cakravarti was the embodiment of sacrifice who offered up his all. Because Bali Cakravarti, who was performing sacrifices, was suddenly pressed down to the nether world in this way, the people were plunged in grief.
Before he went down to the nether world, Bali Cakravarti asked Vāmana for one boon.