Dar'vak wasn't the strongest Jaffa growing up, so he looked for other ways to be of service to his Goa'uld lord. As it happened, his Goa'uld was fairly low-ranking, and thus commanded a barely-terraformed world which needed machines for constant environmental monitoring and control. Dar'vak became one of the Jaffa given the task of maintaining the calibration on these devices, a role which took advantage of his attention to detail. Although this job was presented as a complex ritual, a sacred ceremony demanded of their living god, Dar'vak began to understand the mechanics of what was going on, leading to his realization that Goa'uld powers were merely advanced technology. He continued working for the Goa'uld, however, hoping to learn more and more about their tools to better take them down.
Maintaining this cover, and doing more to make himself indispensable to (and knowledgeable about) the Goa'uld, Dar'vak found himself forced to make some hard decisions and perform tasks that would continue to haunt him. But his perseverance paid off when his world was raided by an SG team. He'd heard of these explorers from whispered conversations and troop movements, and when one showed up, he helped them complete their mission, asking only that they take him with them. After leaving his world, the SG team brought Dar'vak to the Phoenix Site, as a potential candidate for the program.