You are making a 2 stitch cable, instead of knitting into the next stitch after the p1 you go to the 2nd stitch on left needle and knit that. You don't slide it off because there is a stitch in front of it that you haven't worked yet. You then have to turn your right needle to knit into the 1st stitch on the left needle and knit it, then slide both the 1st and 2nd stitch on the left needle onto the right. It causes a cross between the 2 stitches or a mini cable. You get the same result by p1, slip next stitch to a cable needle, knit next stitch and then knit stitch on cable needle. You would want to work loosely in order to not fight the yarn to do this.
Hope this helps.
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