Prop Clip/Wall that blocks props?

[Anonyma]

How do I manage to build a wall that prevents props from breaching it, but allows players to move through it?

I've seen it on a couple maps, such as ttt_whitehouse, but I cannot decompile it to understand just how it does that.
I've never seen a "Prop Clip" texture either.

Could any of you please tell me how to accomplish this?

[Anonyma]

Thanks, I really appreciate it.