The above explanations are fine as to the theory. The only one I had experience was on the old USAF T-33. It assisted in spin recovery, plus made the spin entry more predictable. They were a very rough addition of simply about 18" ( I don't really remember exactly) of angle-iron roughly welded on to the wing leading edge a couple feet or so outside the fuse / wing joint.

Now about a model! [X(] Once upon a time there was a DEMECO (Hal deBolt) model called "Cobra II". Designed for a very old event called Formula II Pylon Racing, with a .40, it made a great sport model with a .50-61 engine.
After a few flights with that machine, I entered a spin. SURPRISE! It went into a beautiful FLAT spin, from which I could not recover. I tried everything I could think of using power and flight controls. It spun right down to the ground, and that trike gear just sat down very politely with the engine at idle, ready to go again. I just had to do that a few more times. On about the 5th or so spin-in, I did finally break a prop.
Then I remembered that old T-33. I rigged some 1/16 x 3/8 x 3" (Size determined by TLAR -- "that looks about right" ) ply strips, made my "angle iron" and glued them about 3" out.
That Cobra then spun very well, easy to recover and just very normal. I removed them once to make another test. Flat and unrecoverable again. Strips back on and all is well again.

I could never figure it all out as others never had such a problem. Must have been a building error on my part, however it was fun. In that model stall strips worked exactly as advertised. I still have another kit. Maybe some day ..........???