DT Human Cycle is a trick that allows character to switch from Human combo transition to DT without costing additional DT gauge. Before game's full release, this trick was very abusable in the demo version that causes dante to gain a lot of DT from the million stab itself.
There are several applications to it that Dante and Lucia can apply:
Example of DT Human Cycle usage with Dante
DT Gain that got changed in the final release