They're called suicide knobs because if you aren't paying attention it can grab you when the wheel is turning back and pull you in to the steering wheel or break your arm, in theory. The name may have something to do with only having one small point of contact with the steering wheel in case something goes wrong, as you won't have much control.
You might want to note that they're illegal in some areas (here in Alberta at least, so I'm told), and you may want to check your local regulations to see if they're legal in your state.
