If you've ever had an item on your bike that just seems to always come loose, you'll be interested in these little gems. The 2-part lock washer uses ramps cast into the opposing faces to prevent bolts or nuts from coming loose. In fact, vibration will tend to make the connection tigher, not looser!
I used these on my highway pegs, which seemed always to turn a bit when I pressed on them. One pair on each side, placed next to the part (bolt or bracket) which was getting unscrewed by my foot pressure, and they no longer move. I could have used two Nord-Locks' on each side to be really sure, but you should know that once torqued down it becomes very difficult to loosen the bolt. It would pay to leave one side with a normal lock washer so you can remove the bolt later if you desire.