Ray, I'm working up more info on that right now, actually.
From this photo you can basically get the concept:
Basically you insert the hook trailer from the back and through the nose of the bait, then you clip the head on and slide the plastic up the head so that everything is enclosed in the plastic. Since the shaft of the hook is free to "flex", the bait takes on a lot more action that with a fixed-shank jighead. Pretty cool concept. The Flex jig was invented by a fella' named Jack Stazo. He's given TTF exclusive rights to manufacturing the head.
Here's a shot from the website of what it looks like rigged:
