My Thoughts
The TSI dual rail frame is designed as a independant suspension, each rail can flex seperatly, this is to absorb imact, the reson we have a bolt on deck plate is to allow for flexability, the reason the rail reenforcment is not welded on the back is to allow the energy to be dispersed down the rail.
The Sissor joint is where the "down tube" is conected to the rails, the DT and rails are machined with a slot to over lap each other, this makes the walls doubled. Twice as strong
The reenforced Down Tube is a doubler strip on the front front of the frame between the head tube and the Nose piece, welded solid, this is the standard for the bike industry. Aluiminum bikes are ALL reenfoced under the head tube on the down tube, as well as most high quality Chrome moly BMX bikes.
As far as I know TSI is the only deck with a reenforced down tube.