Exactly what does "Fast Z Optimization/Clear" do for the Radeon?

[Philly C]

Because when I enable it in Direct 3D and play Half-Life/CS/Firearms I see like transparent textures or something. Little chunks of textures look misplaced or something. Hard to explain. Using 7089 drivers and Direct 3D in Half-Life. V-sync is off. I tried playing around with the Radeon settings and I'm pretty sure the "Fast Z Clear" thing is causing this. Could it be the combination of v-sync off and Fast Z Clear that is causing these "missing" textures? Any thoughts? I'd like to enable "Fast Z Clear" because it seems faster overall.

 

[Deeko]

It's a part of HyperZ, is it not? Apparently, a part that doesn't work correctly lol, sorry, I'm no Radeon expect...maybe someone smarter than me will know