Quoted:
The first time I saw it done was in the '70s, in an article on Israeli tactical training.
I cannot remember the exact reason the technique was taught, but it wasn't a being taught as a technique to be " normally" employed. It was something around a better way to rack the slide, or extreme close quarters, or maybe even as a way to quickly chamber a single round into an empty pistol (as previously noted).
These were still photos in a magazine, so the context of what happened just before and after the picture was snapped was missing, whereas in a video it would have been apparent.
The first time I saw it done in a movie (in the '80s) I remember thinking "that's an improper employment of that Israeli training I saw several years ago".
I know an older Israeli who served probably in the 70's. He still draws and brings the gun into his work space and his free hand on the slide as if to work the action. He does carry with a round in the chamber and he doesn't jack a live one on to the ground, but I guess that is how he was taught and muscle memory is no joke. He is a very good shot and even with this extra hitch in his draw, he is still much faster than me.