I think the main use of a split lock washer is just to add a little axial tension to the system as the nut or bolt is being tightened. This is useful if you cannot get a wrench on both the nut and bolt head, and it helps to get things tightened down in those situations.
A split lock washer probably does bite or gouge into the mounting and/or bolt/nut surfaces a little, but I'm not sure how much it would help with locking and gouging of the surface does not seem a dependable locking method.
If the split lock washer had knurled surfaces or a surface that could "bite into" the mounting surface and fastener faces, I would think that might help a little with preventing loosening. Maybe rough them up a little with a file or something ?
From Unclesyd's link...
"When tension in the assembly is reduced and loosening occurs, it provides resistance to the back-off rotation of the bolt".
I think the statement above is true, but the problem I see is that the vibration, movement, or force that initially overcomes the friction of a properly tightened fastener causing it to loosen, would probably very easily overcome the friction resulting from the axial tension of the split lock washer, which only decreases as the joint gets looser. I guess it depends on the application and what forces are at play but it seems to me that as soon as loosening occurs, the game is basically over that point.
"Technology University have shown that the typical helical spring lock washer exhibits a spring rate after flattening which is approximately 70% more effective than a flat washer of the same thickness".
70% is significant but I would be interested to see how the test was conducted. There can be more friction variation in identical fasteners than 70%, so I'm not sure how they accounted for that in their tests. Did they measure actual fastener tension or stretch in their comparison or did they just torque the fastener down ? I would think they should have tested many fasteners from different production lots and manufacturers.
The shakproof metal components site mentions that the split lock washer is twisted in addition to having the split ends at different elevations. However, whether the washer is twisted or simply has ends with different elevations, flat is flat, and at the point at which the washer is flat (ends at same elevation and twist removed) it would seem to have to act and respond like a flat washer. At the point at which the washer is flat, any difference in spring rate would seem to have more to do with differences in washer material, i.e., elastic modulus of the washer material, etc. as opposed to washer design.
NASA is an authoritative source, but like everyone else their not infallible. I would be interested in seeing actual tests from both sides before forming more rigid opinions.
I'm guilty of using split lock washers for certain things even after reading the NASA report, I guess old habits die hard.