Why did Ambedkar call one article the "heart and soul" of the Constitution?
Because a right you cannot enforce is not a right, and he had spent his life on the wrong side of that difference. Of all the articles in the Constitution, the one he singled out as its very heart and soul was not a grand declaration of freedom but the plumbing beneath it: the article that lets any person go directly to the highest court to have a fundamental right enforced. The guarantees mean nothing, he argued, without a guaranteed way to make a court act on them.
This is the same conviction that runs through his entire career, stated in constitutional form. At Mahad the right to drink from a public tank existed on paper and the tank stayed shut for a decade; the lesson he drew was that a right is worth exactly what the machinery enforcing it is worth. When he came to draft, he made sure the machinery was itself a fundamental right, so that it could not be quietly taken away while the fine promises above it were left standing as decoration.
There is a practical lesson in it. The work is not only to win a good rule but to build the route by which an ordinary person can compel obedience to it. Ambedkar put that route at the centre of the document and told the Assembly, in as many words, that everything else depended on it.