Fairly short read effectively, it’s never been made public what the loophole was/is, and all those who knew first hand are dead now. But it’s speculated to be a few different things, the leading theory (and the one I think is it) is article V, the process by which we amend the constitution. If we can amend the constitution, we can amend article V, meaning we can then make it easier to amend again later, in a downward spiral. We could also amend the constitution, do a bunch of fashy shit, then amend article V to make it so the constitution could not be amended ever again.