Sven, I believe he's using the CRX drive, with the motorcycle frame/swingarm modified for a static rear wheel.
HydroJim, it has to do with center of gravity, how low it is and where it is between the three wheels. In a 2F1R (two front, one rear) format, you typically want it between the front wheels, about 1/3-1/4th the way rearward. The TRex type trikes have barely any weight above the rear wheel, hence their ease of sliding out the back end but rarely tipping over.
I'm following this project closely, I've considered something along these same lines but with a Metro XFI.
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