"Fastback" and "Closed Flex Back" are 2 different types of closed backs.
The "Fastback" is:
The above TJ1952-L prototype had this design to try out. It's suppose makes the glove more sturdy as it is 1 piece. Allows the glove to be longer, but still keep the rigidity. However, I feel you can not get as deep of pocket with this fastback as you can with other open backs.
Open backs have some flexibility to them. Both in the break in process and fielding area. Open backs allow the player to form the a little better than with a fastback. Also, the open back allows the glove to break a little better.
Now, the Closed Flex Back like the PJV16 Frisco posted is sweet in itself. Once you use a Closed Flex Back glove, you can tell the difference between an open back and fastback design and it will be hard to go back to the open/fastback designs IMO. The Closed Flex Back flexes slightly when the ball hits the glove, reduces the impact the ball has on the glove and your hand, slows the rotation of the ball, and then allows the glove to close ALL the way around the ball. After using the Closed Flex Back on a consistent basis, I couldn't go back to an open or fastback glove. It took some time getting use to the closed back feel, but it was worth it. I also have dabbled with the 2 in the pinky and have gone back to straight up after going to the Closed Flex Back.
Take it for what it is worth to you.