It's been since 2006 since the above page was modified, and still longer since the valve was made, and I was always surprised that I hadn't seen another one like it built since (of course I never tried myself, so that's slightly hypocritical)...
Operationally, it's just a barrel sealer (only in a different configuration ((no elbows to bring the tee inline with the barrel)) ), with an attachment on the back which seals off the pilot volume, and moves backwards with the piston, uncovering a large hole to vent the pilot volume quicker.
I doodled up this little ditty (WARNING: large animation) to show how one would function with standard parts and in the configuration of a normal barrel sealer. Does this simplify the concept any?
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Simple barrel sealer, floating o-ring for equalization, 2" tee, 1.7" port... only difference is that the threaded rod going through the piston attaches to an external sealing face which seals the rear port when the piston is closed. The surface area differential keeps the piston and sealing face sealed. Also, the pilot and fill valves would have to be threaded into the side of the tee like the original DFTV.
The animation definitely simplifies it. The diagram on the linked web page is a bit overly complex... quite like mine 
It seems like a very good way to make a piston valve... although I bet it was difficult to tune in the sealing faces to the right spots...
