Flex frame allows more flex and is found in the Ribcor boots, it's aimed at (from what I understand) players that prefer the older style boots before they became so rigid by providing the benefits of the more modern lateral stiffness while also providing a bit more forward flex . Rocket frame is more ergonomic and stiffer all around, less foam I'm guessing to minimize any give/movement between the foot and the structure of the boot. This is my take on it from what I've read and seen... I looked into this stuff a bunch about a year ago before falling in love with the Ribcors... I wasn't a big fan of how stuff and tall feeling modern boots had become, I miss my old Graf 705 feel (didn't miss the weight of them though), I found the Ribcors to be the best of both worlds.