Sinestro, at the time, was the leader of the Green Lantern Corps and regarded to be one of the greatest Green Lanterns ever. He also had a very big ego and believed that he could accomplish what Parallax had failed to do: control the yellow essence of fear without losing himself to it. He also believed that the green essence of will power was no match for the yellow, and that if he wanted to defeat Parallax he would have to use the same power, which is why he asked for a yellow ring to be constructed.

Had Sinistro's plan prevailed he would've worn the yellow ring into battle with Parallax. The latter wouldn't be able to siphon fear from Sinstro's ring because the yellow ring didn't emit fear, it absorbed it, the same way Parallax does.

Sinestro, was the leader of the Green Lantern Corps at the time and regarded to be one of the greatest Green Lanterns ever. He also had a very big ego and believed that he could accomplish what Parallax had failed to do: control the yellow essence of fear without losing himself to it. He also believed that the green essence of will power was no match for the yellow, and that if he wanted to defeat Parallax he would have to use the same power, which is why he asked for a yellow ring to be constructed.

Had Sinestro's plan prevailed, he would've worn the yellow ring into battle with Parallax. The latter wouldn't be able to siphon fear from Sinestro's ring because the yellow ring didn't emit fear, it absorbed it, the same way Parallax does.