I first assumed that that's how they did it, but then it didn't make sense to me because when you insert the seatpost receiver, you can only braze at the top...so how does it stay secure in the downtube?

What does cutting out the actual down tube do, structurally speaking? Does it compromise the strength of the frame? Basically I really like the track look, and if I'm going to build my own frame (first timer...doing it for fun, I don't actually race or anything), I might as well make it look what I want it to look like. But I bought my first fixie last year and have been getting into basic (read: poser) fixie tricks. I need something that will withstand any beating I might give it.

Can someone recommend materials? Is it useless to go for the track look for a fixie that will bear some beating?