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It's not an urban legend, Jason, it's a fact of physics. You're basically throwing sapphire at the glass (it's a ceramic of Al2O3, which is corundum=sapphire) and the tempered glass that they use in windscreens is tough but the high tension under which it's put when tempered makes it susceptible to shattering if it's cracked. Sapphire is much harder than glass (9 c.f 6.5 in hardness) and it only has to create a tiny crack in the glass...the tempering does the rest of the job.
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