I've seen a number of those claims on various sites. The idea is that modern skyscrapers are just too darn hard to demolish, or at least something the size of the Towers would be. So, architects started to build in mechanisms for a controlled demolition as the buildings were constructed. Yes, the Twin Towers were built with explosives (either nuclear or conventional) as part of the design, ready to bring those suckers down "in their footprint"!
Of course, saner minds would bring up that (1) no one has ever heard of this (it's not taught in architecture classes or included in public discussions of civil engineering), and (2) people would be very nervous living and working in huge buildings that could collapse if someone hit the wrong switch. The conspiracists claim that (2) explains (1). It's a benevolent lie. Of course, as soon as the system was used to bring down a skyscraper, it would be out in the open, but that's ... well, let's not go there.
I think the idea is that if they can get their opponents to make what they see as a small concession ("Yes, it would make sense to put a self-destruct mechanism in a giant skyscraper,") it would then be an easy step to get them to admit "And then the Eeevil PTB could possibly have commandeered this mechanism for evil instead of good." Unfortunately, the first concession is so ludicrous this plan doesn't work.