Make 1 Purl Stitch Right (m1pR)
The m1pR, or make 1 purl stitch right, is a right-leaning single increase (when viewed from the knit side of the fabric) that creates a new stitch from the space between two existing stitches.
To work it, use the left-hand needle to pick up the bar between the most recent stitch on the right-hand needle and the next stitch on the left-hand needle from back to front. Purl this new stitch through the front loop (like a normal purl stitch).
This article includes diagrams, and the following video demonstrates how to work a m1pR around the 1:12 mark: