Described simply, follow focus is a technique on video cameras so you can quickly adapt to changing conditions to keep targets in proper focus really quickly. It's the effect you see in movies when someone in the foreground finishes talking, and the camera needs to quickly switch focus to someone in the background. Now you can have that same effect on your DSLR for around $60.The velcro strap with the metal markers are stop points, so you can set the proper focus points for your target and switch back and forth really fast (and accurately) with the handle. If you're feeling super fancy, you can swap focus while zooming in or out, making for that effect Hollywood likes to use when someone's just received incredibly bad news and something even worse is about to happen. You know the one.
If you're good with your hands, velcro and metal clips are cheap, and rigging up a handle for the focus doesn't seem too difficult either.