Ribosomes will bind to the S-D sequence and you will get translation. If I have high levels of tryptophan, I want to shut off even the 1st mRNA for pathway. If I don't have enough trp, I really want to turn on this enzyme. From the codon back is loaded with ribosomes. From the codon forward to the message are no ribosomes.
Ribosomes bind here and go off gleefully making the first message.
If you designed this system. The first protein doesn't have any tryptophan in it. Therefore, you can make the whole protein and the ribosomes will not stall. If you have high levels of Tryptophan, this guy is loaded up past this Tryptophan Codon. This message now falls into a different structure.