I don't like this method. To hold the gun upright while racking the slide, you - or at least I - need to cock your wrist, as in the picture. It works better if you hold the gun on its left side, but if you're clearing a stoppage you need to also roll it to the right side. And if you use it on a CZ, your thumb and finger could very well slip off.
An acquaintance, who has worked with Special Ops and Special Forces troops at Fort Bragg for years, as an instructor, told me a couple of years ago, that they are no longer teaching the "sling shot" method as a standard part of handgun drill. (You still need to know how to TAP/RACK/BANG as a clearance drill, of course.)
The reason: too many troops in Afghanistan and Iraq were having problems with the slide NOT going fully into battery after a mag change, using the sling shop method when the slide was locked back. They now teach using the slide release, using either the strong hand thumb, or the offhand as the last step in the mag insertion drill. They rack the slide using the overhand method when they can.
Whether people acknowledge it or not, either method requires "fine motor skills" when it comes to releasing the slide, and it may be that the sling shot method is more prone to problems under combat conditions, especially with the troops are wearing gloves, etc.
When I was shooting competitively (IDPA) I found that I was just as fast, and more sure of myself, using the offhand to release the slide. And the overhand method when I needed to rack the slide. When releasing the slide I didn't use just one finger of the off-hand to depress the release, but several fingers together, like a claw. A "plus" of this technique: you can do it all without moving the gun away from the target area and you don't have to use your strong hand thumb to release the slide -- some folks can't reach it. The sling-shot requires a dramatic move of the barrel away from the target area.
When I've watched the top shooters in the "combat" games, few use the sling shot method, UNLESS they need to clear a jam. It's not life or death in that context, but it is important to them -- and the results mean dollars. So they do what works best for them.
Do what you want and what works best for you. Confidence is important, too.
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