The threaders I use are by procure and come in a yellow long ziploc bag. They come in packs of two. To answer your question, they do both (slide the line through and slide the worm on the line). It's really hard to explain but I could show you pretty easily first hand if you want to meet for 5 minutes at the highway 512 mcdonalds (I drive by there everyday and seeing you're from spanaway, it would be an easy place to connect). I put a bead above my hook before sliding the worm onto the leader. Unfortunately, the bead doesn't COMPLETELY hinder the worm from sliding down but it helps. I usually go through a pack an outing (but that's some hard fishing too).