How does the 1911 slide remain back after the last round is fired?

A photo in a thread that was posted yesterday reminded me of something I've always wanted to ask about - what prevents the slide from closing after the last round is fired in a 1911.  Is this what does it?  If not, how does this happen?



It seems logical to me that the slide lock could move upwards towards the lock position, and as the slide is closing after firing that last shot, it moves up into the "notch" on the slide that locks the slide in place.

If this is correct so far, what causes the gun to know that it is now empty?  

This is a photo of my Springfield 9mm Range Officer magazine, and to me the logical explanation is that somehow the gun senses the black component that will then be blocking the areas above the "front" facing edge of the magazine.



I've got a follow-up question, but first I'd like to understand this.  Can I assume that this is the way that all modern guns know that a magazine is empty, so they lock back the slide after the last shot?

the tab on the side of the follower engages a protrusion on the inboard side of the slide stop.
it's easy to demonstrate by assembling your firearm without the barrel and inserting an empty magazine.

Look at the gun's slide lock lever from the unlocked breech face side.  There is a portion of it protruding inside the pistol.  When there are rounds in the magazine that portion / lever / slide lock is untouched and so it remains down.  When rounds are exhausted the follower - that black portion you pictured - pushes the lever up and so the slide lock goes into the slide notch.  If you take out the magazine there is nothing pushing the lever up any more and so the slide will not lock back, same way as when there are rounds in the magazine....that is of course unless user pushes the lever up to lock the slide back.

Aha!  Thanks guys, mystery solved:

"that black portion you pictured - pushes the lever up and so the slide lock goes into the slide notch."