Doorknob latch part...purpose?
 

This seems like something I should have figured out by now. The smaller bar that is attached to the latch bolt and moves independently of it, on a door knob latch...what does it do? I've been googling for awhile and can't even find out what it's called, let alone it's purpose. I don't have an extra one to take apart right now so I thought I'd just ask here.

Also, when the door is shut should that part be extended through the latch plate with the bolt or not?

Thanks for any info.

It's an extra lock and it should not extend. If you push it in by itself, the main lock part will not go in. In the movies we see people open doors with a credit card, not with that lock!

Yes. . .confirm it for yourself, Prettygoodist. Try to push in the large bolt while the small one is extended, then while the small one is flush with the door edge.
You don't need to touch the knob.

You can't imagine how many of these are improperly installed.

Thanks guys, it seems so obvious now. I also understand now how an improper installation can negate that feature. Actually I'm thinking most big-box pre-hung doors, with the pre-cut hinge plate, probably get installed in a way that lets that part extend. What's it called? Latch lock?

The strike plate never allows it to extend.