Can you post a link to the pattern, or put the exact instructions in your next post. It sounds to me like you are supposed to be slipping the first stitch that you slipped to the cable needle back to the left needle, then you would knit 2 from the can and then purl the stitch on the left needle. This creates a really hard twist and pulls the edges of the cable in so that it pushes out even more than a normal cable would.
If the pattern really is as you have it listed, then it would make absolutely no sense. If you slipped the 3rd stitch back to the left needle, then there was no reason to slip it in the first place, and if you slip the 1st stitch back to the left needle and purl, then why would you not just purl it from the cable needle. The pattern as you have it here makes no sense. If it's the way I described above then it makes sense to me.