it's a safety feature that uses a plastic "J" shaped key to lock out the firing pin. it's part of the bolt shroud. i was told that sometimes they lock up so i just ordered the old style bolt shroud from Holland gunsmithing . i have a sendero also in 300 RUM and changed the shroud out. i am very happy with it.
Never had a problem with them locking up but the main problem with them is the fact that if you lock them and then go hunting and forget to unlock them and have forgotten to take your J-Key along it can be a case of "Houston we have a problem" Best thing to do is unlock it and hide the key or replace them altogether. Sure saves on the mind stress. Especially if you are a long way from home.
The J-lock shrouds are a bitch to reassemble. If and when you need to take one apart just replace the assembly. Until then, unlock it and throw away the key. It'll serve you well til you get mud and ice in the keyhole.