Peter Gramwrote a review of Yakima SKS Lock Cores - 12 pack on May 5, 2012
These are an important add on to any Yakima racking system. Four cores are needed to lock down a RailGrab tower to start off with. After that, it is often useful to have at least 4 more to lock down other racks on to the rails. Getting a larger set could be worth it for future expansion or for multiple cars, if you want to keep the same lock for all cores.
Installation (and removal) just takes a few seconds per core. There is a key unique to the lock, as well as a control key which allows the lock to be removed if needed, after unlocking it.