What did Ambedkar mean by "constitutional morality"?
He meant a habit, not a document. Constitutional morality is the settled disposition among both rulers and ruled to work the machinery of the constitution — to argue fiercely but within the forms, to accept a result reached by the rules even when you lose, and to resist the temptation to reach past the procedure to grab the outcome you want. A constitution, he warned, is only the top layer; underneath it must run this habit, or the paper is worthless.
He borrowed the phrase from a historian of ancient Athens and turned it into a warning for India. Constitutional morality, he said, is not a natural sentiment: it has to be cultivated, and a people long used to obeying rather than governing cannot be assumed to have it. The forms of the constitution would sit on top of a society that had never practised them, and the gap between the two was where danger lived.
If you are thinking about how to change a system for the better, this is the part to sit with. He is saying that good outcomes taken by bad means corrode the very machinery that would let you take good outcomes again tomorrow. The restraint is not weakness; it is the thing that keeps the rules worth anything the next time you need them.