Center Lock Nut Theory

It's a Center-Locking Distorted Thread Lock Nut ( or reversible locknut ) and they are NOT REUSABLE.
If you take them off you are supposed to replace them.

Supposedly they grip a little better than a nylon insert locknuts... But they are more than likely to cause damage to the external threads they are go onto. But then again, I've had poorly made nylock's strip threads.