It indeed is the e-brake. Personally, I almost never use it, but I have occasionally... sometimes, it works really well for spinning the car around tight courses, because you're breaking almost all of the traction and letting physics do its stuff. For example, try it around the tight hairpins in the Tahiti Maze (that set of tight corner after corner after corner), or try it at that tight hairpin at SSR5 if you suddenly realize that the normal brakes would cease too much of your momentum but aren't strong enough to break traction.