G73 is similar to G71 and G72 in that you define the finish shape in the subsequent blocks (ns to nf). While G71 roughs in a Z (turning) direction, G72 roughs in an X (facing direction), and G73 roughs by following the finish shape.
The manual says that it is used to rough premachined stock, or castings or forgings. I'm willing to bet that 95% of the programmers out there have never used it. IMHO, it's not an effective way to rough a casting.