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.
and this is why people need to be informed of the ultimate legal loophole: the law is simply a collective agreement, if enough people make it clear to everyone else that they think the law is nonsense we can just change it regardless. It's not a magic document enforced by fae.
As far back as high school I assumed this was possible. All it took was learning about amendments 18 (Prohibition of alcohol) and 21 (scratch that).
Those touting their pocket edition US Constitutions (I think we saw Ted Cruz do this?) do the same with bibles. All I see is John Brittle from Django Unchained.