The dark slides have either a black or a gray handle, the gray ones are the ones that lock. I don't have any gray handle ones so i can't tell you exactly how they work, but i think it's only with the later backs.
The locking darkslides have a slot cut into them near the handle. The later backs have a spring loaded button that comes out when the back is removed. When it comes out it locks the slide. When the back is mounted, the button is depressed by the body mounting surface. A black handle slide can be converted to a locking type by carefully cutting that slot into it. It will only lock on a locking type back, naturally.
I do use a digital device in my photographic pursuits when necessary.
When someone rags on me for using film, I use a middle digit, upraised.