Most of what has been written is good advice. However, I disagree on some points.
No matter what nomenclature you apply to a slide stop/release, it is designed to both stop and release the slide. If not, it would not have the external tab enabling the release, eh? I am just going to call it the lever.
The overhand slingshot method is very robust and predictable. It is also relatively slow to complete and reposition the support hand for firing, and it requires two hands.
A properly functioning lever will release the slide with finger pressure when a loaded mag is in place (or no mag in place) as long as it is properly adjusted. As pointed out previously, trying to move the lever down when the follower in an empty mag is adding upward pressure will make it more difficult to push down. But not impossible--when adjusted properly.
It is always good to have options, and being able to close the slide using the slide release lever is a good option to have. The quickest way to do this is by pushing down the lever with your firing hand thumb (single handed use) or the thumb of the support hand as you reacquire a two-handed firing grip. It is faster than using an overhand slingshot method as it requires much less movement. Competitive shooters on a timer almost always use the lever rather than an overhand slingshot.
For me, properly adjusted means the lever will automatically disengage when I insert a loaded mag with enough force. I don't have to finger the lever or slingshot the slide. The slide closes when the mag is inserted all the way. The momentum of inserting the mag actually dislodges the lever from the slide--only when the mag is fully seated and the top round is positioned to load--and it loads just as reliably as either fingering the lever or using the slingshot method but a lot faster. For me, concentrating on defensive shooting, that is desirable.
Other threads have complained that automatic slide closing happened on their guns new from the factory, but there is no downside. It's the fastest way to get a reload back to shooting. I also practice slingshoting, and one hand manipulations. Default = automatic closing; 2nd action = thumb stop down; 3rd action = slingshot.
The point here is this is a simple friction interface between the slide stop notch on the slide and the rear of the slide stop lever. The factory does not adjust this interface. Some guns come through very hard to manipulate. Others come through easier to manipulate. The interface just needs a very little bit of adjustment to work whatever way you want it to.
Unload completely
Remove slide from gun (any gun with a slide stop)
Remove recoil spring and barrel from slide
Locate the slide stop notch on the slide, rear edge
If your slide stop releases too easily, rough up this surface with some 400 grit wet/dry sandpaper. Do just a little--don't try to remove metal, just increase friction by roughing up the surface. Reassemble and test. Redo until it works as desired. You may have to rough up the mating surface of the lever also.
Conversely, if your stop does not release easily enough, simply use some 1000 grit wet/dry sandpaper to smooth this interface and reduce friction. You may need to do the same to the rear engagement surface of the lever also. Even add a dab of grease. Go slowly; test your efforts multiple times until you get what you want.
If it gets too slick, rough it up. You can change this ever so slightly multiple times without hurting your gun as long as you don't use files, take off material, or change the engagement angles.
Enjoy your new Shield. Like all mass produced guns, it may take a little adjusting, polishing and breaking in to reach its full potential.
