The thing in the middle is the draw, it's a scrub/jog wheel. The inner wheel (the part with the finger indentation) lets you scroll kind of like a mousewheel (set to spin freely, no "notches), and it is linked to the timeline of whatever video/audio you are editing. The outer part of the wheel (the rubber part with the hashmark tread) is a different, self-centering knob that only turns a quarter turn in either direction, and works like an analog fastforward/rewind. The farther you twist it, the faster you move in the timeline, and when released it returns to center on its own (unlike the first wheel, which stays where it is when released).
Thus, using these two wheels and the buttons around them, I could move through the video timeline in a dynamic, analog fashion, adding keyframes and performing other functions without having to move my mouse from the element I'm trying to edit.
It's kind of hard to explain if you haven't used one before, but it saves a lot of time and hassle once you're used to it.