To be clear, you should NEVER operate the slide stop lever to release the slide from its locked-open position. There are still instructors out there that teach that, but many do not anymore. Every time you want to release the slide from its locked-open position, rack it. There is no need to fiddle around with the slide stop lever. There is also no need for learning two different procedures (pushing the lever down or racking the slide) for the same action. The main reason why instructors have moved to this is to simplify the operation of a firearm. Racking the slide works on every semi-automatic handgun and with both hands, whether there is a slide stop lever or not. Some instructors may argue that this is a gross vs. fine motor skills issue, but for me this is a pure "consistency so it works under stress" issue. Have you ever given the thought that I may need to operate my "right-handed" handgun with my left hand, such as when I am injured? I would run into the same issue as a righty that lefties constantly complain about when it comes to the slide stop lever. Watch this instructional video to understand what I mean:
James Yeager | How to Reload Your Pistol and Rifle - Part 1 of 3
PS: Note that the slide stop is sometimes referred to as the slide lock, which adds more confusion. On a Glock and a S&W M&P, the slide stop keeps the slide locked open when the magazine is empty. On a Glock there is an additional slide lock that is operated to disassemble the firearm.
